If breastfeeding which fulfils the conditions took place, namely five or more feedings within the first two years, then the relationship of mahram is established, and the one who was breastfed becomes a mahram of the one who breastfed him, as do her daughters, mothers and sisters. The brother of a woman through breastfeeding is a mahram to her, and it is not permissible for the husband to deny this relationship of mahram, or to forbid his wife to greet her brother through breastfeeding or to talk to him, because the Prophet (peace and blessings of Allaah be upon him) said: “What becomes mahram (forbidden for marriage) through breastfeeding is that which becomes mahram through blood ties.” 

This applies unless there is the fear of fitnah, such as if that brother through breastfeeding is not righteous and is not to be trusted with matters of honour, and there is fear that he may mistreat his sister through breastfeeding. In that case she should not be allowed to meet him or be alone with him. End quote. 

Shaykh Saalih ibn Fawzaan ibn ‘Abd-Allaah al-Fawzaan (may Allaah preserve him).