This is the first time i am hearing that jewelery needs to be passed on to the MIL. Maybe its something practised in some places only?
I've never heard of this before.
But I have to say, what's the big deal? Are you really marrying into a family that you mistrust so much? It's one thing if you were afraid kids would get into your such, but to fear your MIL? What will she do?
I keep my gold jewellry in a locker at the bank. If we're traveling or something, my mom has it or my MIL has it. It's not a big deal.