if someone had stolen money from relatives (not only father or mother) & he wants god to forgive him. what would he do? knowing that he does not know the amount of money he had stolen exactly

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Stealing is forbidden regardless of whether it is done to relatives or alien people.

So, any person who steals should repent to Allah seeking forgiveness from his sin. He should also regret his shameful act and have firm resolution never to do this act again.

Moreover, his repentance cannot be complete unless he does his best to return what he has stolen to its owner. Now, if he does not know the exact amount, he should give an amount that makes him feel assured to have returned at least what he had stolen. Only then would he fulfill a true repentance from this sin.

Allah knows best.