All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Probably, the questioner understood that a foster mother takes the same rulings of a real mother regarding the matter of inheritance. This understanding is not correct. There is no inheritance due between a foster mother and her foster child.
As for what you mentioned that suckling from the relatives is forbidden, we think that you did not understand the meaning behind the expression of the scholars. They mean that what is made forbidden by the reason of lineage (due to birth) is also forbidden by the reason of the relation based on suckling. So, every woman that is forbidden due to lineage she is also forbidden due to suckling i.e. mothers, daughters, sisters, maternal and paternal aunts, daughters of a brother and a sister.
Therefore, if a child drinks milk of a woman then she and her daughters become forbidden for him. Thereupon, he cannot marry one of them. However, suckling is permissible so, a woman can suckle any child and then he becomes her foster child due to suckling from her.
Finally, we advise the questioner brother to study the Fiqh books with the help of scholars and ask them about every matter that is not clear to him.
Allah knows best.