Oh Dear mggt,
You poor thing! I really feel for your situation and how hurt, angry and frustrated you feel! This is such a difficult situation for you. I see that you and your H wanted to do the right thing by helping your DD and GD, even though you instinctively knew it was probably not a good idea. Your heart is in the right place. AND I know you did the best you could in the moment, and you made the best decision you could at the time. I'm so sorry the situation went South so quickly! What's that old saying? "No good deed goes unpunished."
I think it's good you took a break at your sister's place for a few days. You needed it! (I just did the same thing.

We ALL need a break sometimes.) With that said, I would urge you NOT to make any permanent changes (like getting your own apartment) until you've had time to cool off and think this through. I'm sure you know making decisions in the heat of anger are not our best decisions.
I know how hurt and angry you are by your husband's invalidation of your feelings and needs in this situation! It feels terrible to be blamed for our DD's BPD rages. I know when I'm angry and hurt, it is impossible for me to consider that I could have possibly had a different outcome. My DD17 has BPD, and my H has Aspergers, so he is emotionally clueless sometimes. I know they both do the best they can, but I still get furious when my H tells me, "You're not helping" right in front of our DD. She takes that as ammunition to use against me, which makes things worse between us. Like you, I have retreated to my room many a time to stay out of her way until I cool off.
Are you or your DD in therapy? Would your H be open to a bit of couples counseling to support YOU through this difficult time? It's true that BPD is a mental illness... .one of the worst. So it only makes sense that those of us who love and care for someone with BPD need a lot of extra validation, coaching, and support from time to time.
I have been in your shoes many times! What can you do to nurture and care for yourself and go "lick your wounds" so you will be ready to continue on the very difficult journey of supporting your DD and GD? Maybe you need a bit more time at your sister's? Can you get some one-on-one counseling to help you process through your hurt feelings?
There are no easy answers or quick fixes for the challenges we face dealing with our BPD loved ones. It's a long journey we're on. The tools and validation DO work, but I've discovered I have to be in control of my own emotions to use the "tools" effectively with my DD. It's certainly harder to validate someone who isn't listening to you and doesn't give a rat's a$$ about your feelings. We need to feel heard and understood too! But that's what THIS site is for. I certainly can't look to my emotionally challenged family to be validated, so I make sure I have a good support system outside my family.
Hang in there! Keep breathing and know you are not alone. When your anger and frustration passes, you will be able to see the situation from a clearer place and make decisions about your future then. In the meantime, know that you are not alone!