I’ve been reading about how to handle people with BPD and it says to give lots of support and validation and show you care.
That advice is when you are in a relationship and trying to repair it so it can work. If you have decided the relationship, the marriage, will have to end, then you of course your prime goal is to protect yourself and your child and proceed on the path toward unwinding the relationship. You have no obligation to subject yourself of the children to seriously bad behavior.
Be aware that courts are reluctant to block the misbehaving parent from parenting altogether. Depending on the level of abuse or discord, the court may issue a temp order to give some distance apart and structure for how the kids have visits, how long, whether overnights, etc. If supervised visits are ordered, even temporarily, then it would be best for you that a neutral but trained professional do the supervision. You won't want to do it and probably you won't trust his relatives to do a good job of it.