someone has to talk to her. it sounds like this is a pattern of her whole life; she doesnt respect anyone else's boundaries. it sounds like your husband's whole life was controlled and disrespected by her. always her forcing her way on everyone else.
this is HORRIBLE behavior, and its sad that your husband even as a grown adult cant just suck it up and stand up for you guys.
whos cabin is it? is it the brother in law's ? what is his view on it? one way or the other, SOMEONE has to talk to her and say "hey, we just dont have room". this is really not fair to you or anyone else. its just going to make vacation a torture, you are going to be cranky, so then that affects everything else, including how you feel about spending time with your daughter and everyone... i mean, holy cow, i know how you feel!!!
my husband's mother is similar to this. when i read this, i could literally imagine myself going through the same thing. when we go to see his niece, they always think they have to go also. we have finally gotten to the point where we KNOW we enjoy the time with his niece much better when shes not with us.
you just have to be firm. and hopefully your husband will at least back you up. you dont want him caving because of the pressure she puts on him, so you really need him to be SURE he supports you and your opinion and feelings. this is a HUGE disrespect of your boundaries, and everyone else's too. i mean, seriously? she thinks its ok to just call BIL and say "im coming with" without even asking!?!?! seriously? this is NOT cool.
holy cow. you need therapy. not because you are doing anything wrong, but because you can be darn sure SHE wont go to therapy, but you and your husband need to know how to deal with her; how to lay concrete boundaries and not let her violate them.
im so sorry you have to go through this, but i empathize because i know the frustration, i know the anger, i know the feeling of just wanting to give up, lock the doors, and turn off the phone.
im guessing just having a baby is another time where she forces herself into your house and life too huh? man, my MIL would not leave me alone when i had my son, and it really destroyed our relationship. she couldnt respect me enough to leave me alone with my son, and she couldnt even let my husband - her own son - comfort our son. i handed him to my husband once, and she rushed over to him and TOOK OUR SON OUT OF HIS ARMS. my husband just stood there with his jaw dropped, and his heart totally shattered. he was just trying to be a good dad, and it was a big adjustment for him to begin with... and then she undermined him like that... it was a big blow to him.
anyway. im kinda just blabbing
im so sorry. i hope that you can find a way to stand up for what you feel, and that your husband at least would back you up and stand behind you. he needs to do this. he is NOT under his mother's control unless he LETS her control him. as an adult, its time to step up and be the man of your house, and be on YOUR team and not fear her.