I am a Muslim Brother.
(1) Is my mother mahram or non mahram to my step daughter’s husband? (Both perspectives man and woman.)
(2) Is my mother mahram or non mahram to my foster daughter’s husband? (e.g. child that was breastfed while under the age of two.) (Both perspectives man and woman.)
In the name of Allah the most Gracious the most Merciful
The answer to your question is as follows:
For your mother the husband of your step daughter is a Ghayr-Mahram. It is necessary to observe hijab from him. Observing hijab is not necessary from the husband of your foster daughter. As there is no commandment of observing hijab from the husband of the real daughter. And the hadith says: “What becomes unlawful (for marriage) through breast-feeding is that which becomes unlawful through lineage.” So as the real son-in-law is a Mahram in the same way the foster son-in-law will also be a Mahram.
And Allah knows best
Darul Uloom Waqf Deoband