What is the ruling on money stolen by the questioner as a child, and is it obligatory to inform the sister's husband of the stolen amount despite the family problems that would result from that?

Stolen money must be relinquished, even if it was acquired before puberty, by returning it to its owners or seeking their forgiveness. If informing them would lead to harm, then do not tell them. The money remains a debt on your conscience, to be returned to them when you are able.