Welcome normalita,
I do not have experience in dealing w/what sounds like a very nasty family situation. I just wanted to welcome you here and am sure others with be along shortly with some experience, strength & hope (E S & H.)
Just a few thoughts I had:
Sounds like the mis-delivered order was all just a bad mix up, I wouldn't waste any more of your time stressing over it. Remind workers that any time they have to go to the brother's place of business to leave things cleaner than what they found it and put the rest of it out of your mind.
As for the M-I-L, won't she stay with the brother? You may know about the visit, but it doesn't have to concern you.
You could write "return to sender" on future letters from her.
Good luck and I'm glad you found a place to vent some of your frustration.