She has brothers through breastfeeding, but her husband does not acknowledge the rulings on breastfeeding
Question

                A woman has brothers through breastfeeding. Her husband tells her not to greet them or talk to them at all justifying this by saying that he denies the breastfeeding brotherhood issue. What is your opinion on this?.
Praise be to Allah.
    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).