I run an Islamic group for women and from time to time sisters ask questions which I believe are for the scholars to answer I advice the sisters about this and they give me permission to send there questions please could you answer this question for this sister.
Question: from sister:
My friend is in a situation where she has been married to a relative for a little more than two years. I don't know how or why this happen to her but she found out that they are both fed by the same woman as babies. They both are from Somalia and know each other from family ties. I don't know how this just came up or how they never knew about this, she told me is common to have one woman feed several other babies from other people. When I read online that two people are considered family if they have the same woman that fed them they can't get married. The problem is after them both found out about that she continued to stay married with brother although I told her about it. She said she been with him so long she can't see herself to leave him and marry someone else. She said she already has two sons by him and is in her mid thirties and is too old to find a new husband. What should I do after I already tried to convince her, should I just watch as I know its wrong and they are very happy as a family. They are both practicing and seem fine together?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

If it is confirmed that this woman had breastfed along with this man from the same woman 5 times or more, then she is considered as his sister by suckling relationship. Therefore, it is an obligation to invalidate this marriage contract and it is not permissible for her to let this man have sexual relationships with her, and she should not tolerate this matter otherwise their being together is nothing but fornication. Fornication with a relative is a much greater sin than that committed with a non-relative. However, if there are children from this marriage, they can be attributed to the father because at the time of marriage they thought it was permissible.

This is of course if the suckling of this man and woman from the same woman is confirmed, but if there is any doubt about suckling from the same woman or about the number of times they suckled, which renders a person forbidden to marry the other, then it cannot be confirmed that it is forbidden for him to marry her.

Nonetheless, to be on the safe side, it is more appropriate to invalidate the marriage contract because of the existence of doubt. If this man divorces her, he should not stay in seclusion with her like a Mahram (a non-marriageable relative) and should not look at her and the like, just to be on the safe side.

Lastly, it should be noted that it is an obligation to be very serious about the matter of suckling and not undermine it, because if it is confirmed, then it renders a person being a Mahram to another, while not being sure about it or forgetting it and the like [the matter which later on leads to many problems, e.g. when being married to each other and so forth].

Allaah knows best.