If an infant sucked milk just once (tasted 2-3 drops of milk) from a lady then does she became his foster mother and when the baby is older, does she become mahram to him? Also does that lady’s children become like brothers or sisters to this infant?
In your situation, because the baby sucked only a few drops of milk, this does not make the baby mahram to the woman or to her daughter. In other words, the woman is not his foster mother, and her daughter is not his foster-sister.
For more further rules related to adoption, please read: Rules of Adoption in Islam
Answered by: Sheikh Mansour Leghaei