Sheikh Faysal Mawlawi, Deputy Chairman of the European Council for Fatwa and Research, states the following: “The Prophet, peace and blessings be upon him, is reported to have said:
“Fostering makes prohibited for marriage men and women who are prohibited through lineage.”
It is clear from the above Hadith that foster sister is just like a real sister. A real sister is not a Mahram for her brother-in-law, for he can marry her after her sister’s death or divorce.
Thus, such prohibition of marriage is of temporary nature, i.e. a person is only prohibited from marrying his sister-in-law so long as he holds her sister (i.e. his wife) in marriage.
Based on this, foster sisters are still strangers to a husband, and he cannot deal with them as Maharm (unmarriageable relatives), especially after they attain the age of maturity.