Breastfeeding with the aim of establishing the mahram relationship

There is nothing
wrong with your wife breastfeeding her sister’s daughter and her sister
breastfeeding her son. If there are five breastfeedings and that is done in
the first two years of life, the girl will become a sister through
breastfeeding to all of your and your wife’s children, and your son will
become a brother through breastfeeding to all of the children of his
maternal aunt and her husband. 

The breastfeeding
that establishes the mahram relationship must meet two conditions: 

1.

That there should
have been five or more breastfeedings, because of the hadeeth of ‘Aa’ishah
(may Allah be pleased with her), who said: Among the things that were
revealed of the Qur’aan was that ten definite breastfeedings make a person a
mahram, then that was abrogated and replaced with five definite
breastfeedings

Narrated by
Muslim, 1452 

2.

That should be
within the first two years of the child’s life, because it was proven that
the Prophet (blessings and peace of Allah be upon him) said: “There is no
breastfeeding except that which fills the stomach.”

Narrated by Ibn
Maajah, 1946; also in Saheeh al-Jaami‘, no. 7495 

Al-Bukhaari (may
Allah have mercy on him) said in his Saheeh: Chapter on one who says
there is no breastfeeding after two years because Allah says (interpretation
of the meaning): “The mothers shall give suck to their children for two
whole years, (that is) for those (parents) who desire to complete the term
of suckling” [al-Baqarah 2:233]. 

There is nothing
wrong with breastfeeding children with the aim of establishing the mahram
relationship between them, because this is a permissible aim and there may
be a need for it. The Messenger (blessings and peace of Allah be upon him)
instructed the wife of Abu Hudhayfah to breastfeed Saalim so that he would
become a mahram to her. 

And Allah knows
best.