Family drama. Need advice!

My sister has a ten year old daughter and she is no longer with the girls father (my ex BIL). He turned out to be manipulative and controlling. But my sister lives in the finished basement of her ex’s new house so they can coparent their daughter (the parents share no living space together and do not see each other at all day to day). The thing is: his family has lots of money and so he basically gives my sister money to live and pays for everything (my sister does work but she’s a yoga teacher and makes very little money.)

My two year old daughter and I are supposed to be flying out there to visit. My mom is also flying out from her state to visit at the same time as well as we can all be together.

Well, turns out that my ex BIL is livid that I will be there for nine days, when he only “approved” 7 (I had no idea there was a time limit on how long I could be there). He plays it that now my niece won’t be able to spend lots of time with her Nana (my mom) and THATS the issue - he says that these are really my nieces wishes. But my mom will be there for 8 days longer than I will. Then we come to find out that doesn’t seem to really be the case. My niece wants to see us but “has a hard time advocating for herself” feels like she “isn’t heard” when me and my daughter are around and that she feels more heard when it’s just her mom and Nana. It should be noted that I haven’t seen my niece in over a year and we all haven’t spent time together as a family in two years. Also, what this basically translates to is that my niece doesn’t always get what she wants when there are more people around. My sister and mom are more likely to give her whatever she wants and allow her to dictate what we do/don’t do each day. My niece has been abroad with the paternal side of the family for a month so no one knows how she really feels. My sister talked to her briefly on the phone but it’s hard to tell what my nieces true feelings are, and what she’s just repeating from her dad.

Now TBH: I know my ex BIL is teaching my niece his manipulative/power/control techniques because I’ve seen it in action plenty of times. I hear about it often, and she often plays her parents against each other.

So my ex BIL is demanding that I leave two days early or go to an air BnB (money isn’t an issue for him, he’ll pay for the hotel or change in flights) for two days so I would be within his approved seven day limit.

I’m disgusted by this entire scenario and think it’s incredibly over dramatic. My mom thinks the same way too. We thought about not going at all but we are excited to go and spend time as a family and we don’t want to damage our relationship with my niece/her granddaughter by not showing up (we don’t get to see her often and family is important to us). But we want to set boundaries around what we deem as acceptable behavior (not these demands of a controlling man and whims of a ten year old). And what difference does TWO DAYS make anyways?

I don’t know what to do. If I stand firm and tell my ex BIL “too bad”, I’m coming for nine days - he’ll make my sisters life a living hell and I don’t want to make it any harder for her than it already is. My sister won’t stand up for herself because it’s too much stress for her.

I don’t really want to go to an Air BnB or hotel for two days because honestly, I’m hurt. I feel like I’m being banished or something. And I’m angry that my niece is being raised in such a way that she gets whatever she wants (I know there’s nothing I can do about that). I could leave two days early but I feel like that’s kind of giving in. But why should I stay where I’m not wanted? Although my mom really wants us to stay the whole time because she doesn’t get to see us often either and she’s getting old, and we should all consider what she wants as well, no?

What would you do in this scenario?