Can Islam permit my son to call my sister or my wife’s sister mummy or my son to call my brother or my wife’s brother in law daddy?

Answered according to Hanafi Fiqh by

بسم الله الرحمن الرحيم

(Fatwa: 1224/926/B=9/1434)

Your son should call your sister ‘phuphi’ and your wife’s sister ‘khala’, he should not call them ‘mummy’. Likewise, he should call your brother ‘chacha’ (uncle) and your wife’s brother ‘mamu’. He should not call them ‘daddy’.

Allah knows Best!

Darul Ifta,
Darul Uloom Deoband