WR, I would leave things be on FB for a while. It's evident that there is a mind playing game going on. One that has him and his family messing w/your head, heart and feelings. Continue as you have been until after everything has been finalized. Then, and only then, if you want to do something different on FB do it.
As for your inlaw wanting to friend your mother, well that's just plain crazy right now. There's no telling why she wants to be friends on FB w/your mother, but you never know...she may be wanting to gain info from your mother or to see what you and your mother have been posting.
Don't join in on your inlaws' games. Stay the course. Your h is a very desperate man and he will do anything and use anyone to his advantage to get what he wants. I don't trust any of them at this point in time. Be careful what you share on FB and w/your inlaws from this day forward. I know it's difficult not to chat w/SIL #2, but you've got to be careful because you do not know if she's telling your h or her family what you say.
Stay the course and continue as you have been. The truth will eventually come to light.
Enjoy the festivities tonight and tomorrow is a new day and a new year!
Happy New Year!
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.