DO NOT give any person a copy of any of your idenity papers, passport or otherwise.  That is something you make a certified copy of yourself and put it in a safe place in case you need it, such as a secure place in your home, a safe, or even a safe deposit box at your bank.  Those papers are way too important for you to be sharing them for any reason.  I'm going to assume that you don't trust this relative of your husbands, or you wouldn't be asking this question at all.  That's your first clue NOT to do as they ask, trust your instincts.