Praise be to Allaah.
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).