I think we all have that one inlaw that is a pain. Mine is my SIL too. She is so nosy and bossy and always jumps to conclusions. I used to just take it and not say anything for the sake of the family but then I started to speak my mind and it brought fights up with my hubs and I (it's his brothers wife so his SIL too). I will vent about her now to my husband and ignore all her behavior. What makes me the most mad is that whenever she throws a party/get together she invites you and if you plan on coming then she TELLS you that you need to bring XYZ. Now this is okay if the understanding is that it is a potluck, but when I was told I needed to bring XYZ to her childs birthday party on top of a gift I lost it and said NO MORE. I will NOT attend any more parties she has because of this. I was raised that if you hosted an event, especially a birthday party that you supplied everything and all everyone else had to do was show up and have a good time. Sounds like you SIL is wayyyy worse and I would have lost my mind by now.
Hopefully you can talk to your brother and let us all know how it goes!!!