I had to deactivate my FB account and start over with a different email address too. On the second account, I only invited a few people I like to connect with. When other people send me friend requests now I decline or ignore them.

It may seem harsh, but I had other issues with FB similar to yours. Not in-laws, but other negativity issues so I just started over with firmer boundaries.

You absolutely have the right to set boundaries for yourself, even on FB!

Good luck!