Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
Your husband is permitted to teach your foster sisters how to play tennis as long as they are below the age of puberty, but after that he is not permitted to do so as he is not one of their Maharim.”
In his response to the question, Sheikh Faysal Mawlawi, the late Deputy Chairman of the European Council for Fatwa and Research, states:
The Prophet (peace and blessings be upon him) is reported to have said:
“What becomes unlawful (for marriage) through breast-feeding is that which becomes unlawful through lineage.” (An-Nasa`i)
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, your two foster sisters are still strangers to your husband and he cannot deal with them as maharm (unmarriageable relatives), especially after they attain the age of maturity.
However, it seems from the question that your two foster sisters are still young. If this is the case, then there is nothing wrong if your husband teaches them tennis. But, after they have reached the age of puberty, he cannot continue teaching them tennis.”
Almighty Allah knows best.
Editor’s note: This fatwa is from Ask the Scholar’s archive and was originally published at an earlier date.