Firstly: 

Every Muslim has to believe in all the hadeeths of the Messenger (peace and blessings of Allah be upon him) – if they are saheeh (authentic) – and not reject any of them, because his hadeeths and his Sunnah (teachings) are revelation (wahy) from Allah. Whoever rejects the hadeeth of the Messenger of Allah (peace and blessings of Allah be upon him) has rejected revelation from Allah. 

Allah says (interpretation of the meaning): 

“By the star when it goes down (or vanishes).

Your companion (Muhammad) has neither gone astray nor has erred.

Nor does he speak of (his own) desire.

It is only a Revelation revealed.

He has been taught (this Quran) by one mighty in power (Gabriel).

One free from any defect in body and mind then he (Gabriel in his real shape as created by Allah) rose and became stable.” [53:1-6] 

Allah has commanded the people to obey His Prophet (peace and blessings of Allah be upon him). He has enjoined this in many verses of the Quran, of which we will quote some. Allah says (interpretation of the meaning): 

“Say (O Muhammad): “Obey Allah and the Messenger (Muhammad).” But if they turn away, then Allah does not like the disbelievers.” [3:32] 

“He who obeys the Messenger (Muhammad), has indeed obeyed Allah, but he who turns away, then we have not sent you (O Muhammad) as a watcher over them.” [4:80] 

“O you who believe! Obey Allah and obey the Messenger (Muhammad), and those of you (Muslims) who are in authority. (And) if you differ in anything amongst yourselves, refer it to Allah and His Messenger, if you believe in Allah and in the Last Day. That is better and more suitable for final determination.” [4:59] 

“And perform As‑Salah (Iqamat‑as-Salah), and give Zakah and obey the Messenger (Muhammad) that you may receive mercy (from Allah).”
 [24:56] 

And there are many similar verses. 

The one who rejects the Sunnah is a kafir and an apostate. 

Al-Suyooti (may Allah have mercy on him) said in his essay Miftah al-Jannah fi Ihtijaj bi’l-Sunnah: 

Note that whoever denies that the hadeeth of the Prophet (peace and blessings of Allah be upon him), whether it describes his words or deeds, so long as it meets the conditions outlined by the scholars, may be quoted as evidence, is a kafir and has gone beyond the pale of Islam; he will be gathered with the Jews and the Christians or whomever Allah wills among the kafir sects [i.e., on the Day of Resurrection]. 

Those who want to restrict themselves to the Quran only are called al-Quraniyyoon. This view of theirs is an old view which the Messenger (peace and blessings of Allah be upon him) warned against in more than one hadeeth, as we shall see below. Among the soundest evidence that this view is false is the fact that those who say this do not really follow what they say. 

How do these people pray? How many times do they pray each day and night? What are the conditions and details of zakah? What is the nisab (threshold of wealth) for paying zakah? What is the amount that must be paid? How do they do Hajj and ‘Umrah? How many times do they circumambulate the Ka’bah? How many times do they go back and forth between al-Safa and al-Marwah? 

There are many other issues for which the details are not narrated in the Quran, rather they are mentioned in the Quran in general terms, and the Messenger (peace and blessings of Allah be upon him) explained them in detail in his Sunnah. 

Would these people refrain from acting upon these rulings because they are not narrated in the Quran? 

If their answer is yes, then they have passed judgement against themselves that they are kafirs, because they have denied a basic principle of Islam that no Muslim has any excuse for not knowing and on which there is unanimous consensus among the Muslims.  

If they reply that they do not refrain from following these rulings, then they have demonstrated that their view is false. 

Shaykh al-Islam Ibn Taymiyah said – after quoting the verses that enjoin following the Messenger (peace and blessings of Allah be upon him) –  

These texts enjoin following the Messenger (peace and blessings of Allah be upon him) even if we do not find what he said specifically referred to in the texts of the Quran. These verses also enjoin following the Quran even if we do not find what is said in the Quran specifically mentioned in the hadeeth of the Messenger (peace and blessings of Allah be upon him). 

So we must follow the Quran and we must follow the Messenger (peace and blessings of Allah be upon him). Following the one implies following the other, for the Messenger (peace and blessings of Allah be upon him) conveyed the Book, and the Book commands us to follow the Messenger (peace and blessings of Allah be upon him). The Book and the Messenger (peace and blessings of Allah be upon him) do not contradict one another at all, just as the Book does not contradict itself. Allah says (interpretation of the meaning): 

“Had it been from other than Allah, they would surely, have found therein many a contradiction.” [4:82] 

And there are many hadeeths from the Prophet (peace and blessings of Allah be upon him) which state that it is obligatory to follow the Quran and that it is obligatory to follow his Sunnah, for example, the hadeeth in which he said: “I do not want to find any one of you reclining on his pillow, and when he hears of something that I have commanded or forbidden, he says, ‘Between us and you there stands this Quran, whatever we find is permissible in it we will take as permissible, and whatever we find is forbidden in it we will take as forbidden.’ For I have been given the Book and something like it with it; it is like the Quran or more.” This hadeeth is narrated in the books of Sunan and Musnad from the Prophet (peace and blessings of Allah be upon him) with a number of isnads (chains of narration) from Abu Tha’labah, Abu Rafi’, Abu Hurayrah and others. 

In Saheeh Muslim is it narrated from Jabir that the Prophet (peace and blessings of Allah be upon him) said in his Farewell Sermon: “I am leaving behind among you something which, if you adhere to it, you will not go astray after that. It is the Book of Allah.” The version narrated by al-Hakim says “The Book of Allah and my Sunnah.” (Classed as saheeh/authentic by al-Albani in Saheeh al-Jami’, 2937). 

In al-Saheeh it is narrated from ‘Abd-Allah ibn Abi Awfa that it was said to him: Did the Messenger of Allah leave a will? He said, No. It was said, How then is it prescribed for people to make wills when the Prophet (peace and blessings of Allah be upon him) did not make a will? He said, He made a will in which he enjoined (adherence to) the Book of Allah. 

(Narrated by Muslim, 1634) 

The Sunnah of the Messenger of Allah explains the Quran, as it explains the number of prayers, how much should be recited in them, which should be recited out loud and which quietly. It also explains the amounts of zakah to be paid and the threshold at which zakah becomes due; the rituals of Hajj and ‘Umrah; how many times one should circumambulate the Ka’bah, go between al-Safa and al-Marwah and stone the Jamarat (pillars), etc. 

When any Sunnah is proven to be saheeh, the Muslims are agreed that it is obligatory to follow it. There may be something in the Sunnah which a person thinks appears to go against the apparent meaning of the Quran and add to it, such as the Sunnah which explains the threshold of stealing at which the hadd (Islamic legal) punishment becomes due, and the Sunnah which stipulates that the married adulterer is to be stoned. This Sunnah must also be followed, according to the view of the Sahabah/Companions (may Allah be pleased with them) and those who followed them in truth, and all the groups of Muslims. 

Adapted from Majmoo’ al-Fatawa, 19/84-86 

That which was brought by the Messenger (peace and blessings of Allah be upon him) is true just as the Quran is true.

 Secondly: 

You should not forsake your family; rather you should treat them kindly and strive to call them to follow and accept the Sunnah. 

Allah says (interpretation of the meaning): 

“Invite (mankind, O Muhammad) to the way of your Lord (i.e. Islam) with wisdom (i.e. with the Divine Revelation and the Quran) and fair preaching, and argue with them in a way that is better. Truly, your Lord knows best who has gone astray from His path, and He is the Best Aware of those who are guided.” [16:125] 

“And We have enjoined on man (to be dutiful and good) to his parents. His mother bore him in weakness and hardship upon weakness and hardship, and his weaning is in two years; give thanks to Me and to your parents. Unto Me is the final destination.

But if they (both) strive with you to make you join in worship with Me others that of which you have no knowledge, then obey them not; but behave with them in the world kindly, and follow the path of him who turns to Me in repentance and in obedience. Then to Me will be your return, and I shall tell you what you used to do.” [31:14-15] 

And Allah knows best.