Parents in law moving next door to bpd sibli

[sisinlawstruggle]

I am looking for help from people who understand. A little background. My sister-in-law has BDP. She was diagnosed in her teens. She was married and quickly divorced. She married for a second time and had a child when I married into my husband's family, her brother. Her second marriage was particularly horrific and she ended up living with my in-laws for 7 years after that marriage dissolved. During that time I observed her abusing my mother-in-law in a number of ways, verbally emotionally mentally.
Fast forward a few years, sister-in-law moved across the country from us to where my in-laws would eventually move a year later.
 About 6 months ago our in-laws dropped a bomb on us that they were planning to build a house with sister-in-law and her child and her now third husband.
The other siblings in the family three brothers all planned a talk with their parents about this trying to prevent it, but before we could have a real conversation about it the parents dropped the plans.
Which brings me to now, They have decided to build houses right next door to each other.
This is extremely difficult for me because having my sister-in-law live next door to my in-laws is not a safe situation for us to visit my in-laws.
My primary question is should I/my husband express to my in-laws that when this happens it will no longer be a safe place for us to stay with them or visit them in their home. I cannot prevent them from doing this nor do I want to sound like I am manipulating them by saying if they do this we won't ever visit them. I also feel like they might want to know that information before it happens? I think they already know we don't particularly approve of their plan although they have never specifically asked our advice.
I do not think we would have a hard time expressing that boundary if when this move happens, wondering all of your thoughts about communication particularly before.
We tried to get together over Christmas but sister-in-law had a huge freak out and was very verbally abusive, over phone and text, to another sister-in-law.
Parents are really big on all kids being together, two of the children live in the same town as parents and the other two live very far away.
So should we express what our boundaries will be should this happen?
I'm feeling so frustrated because borderline sister-in-law is totally playing on mom's fears and a manipulative way, "when Dad dies we can be there to take care of you so you won't be alone" (dad is only 3 years older than mom and in good health).
Slightly related question. I need help figuring out how to ask my mother-in-law to separate her daughter from her relationship with us. When my husband's parents come to visit us things are usually fine, however regularly throughout the visit my mother-in-law sends her daughter pictures of what they're doing with our kids and is having a text conversation with her throughout the day about how their time here is going. It's like my sister-in-law thinks there are three adults in the family her and her parents and her brothers are all the children. I want to separate a relationship with her from our relationship with my husband's parents, but because of the weird interconnected dynamics between my mother-in-law and her daughter I'm struggling to figure out how to do that. My mother-in-law is definitely enabling and does not have good boundaries with her daughter.
Any advice?