Hi Zabava,
I haven't had a chance to welcome you to the board yet, so please consider this a welcome!
This is definitely a challenge for any parent with children as you consider going back 'home' to visit. The children can help to be a distraction or a buffer, and I wonder if you feel that will help to dispel some of the tension? Of course you will need to watch out for them first of all. How long do you plan to stay? What would you consider a safe time that may not enter into the drama time?
One thing my T has told me over and over when I think of going to visit my uBPDm's home (she passed away a few years ago): you don't ever have to go back there again. My step dad earnestly wants me to stay with him, yet the entire house is still decorated and filled with my mom's presence, and it is very difficult for me to go there. I have chosen to stay a couple of times, but under this rule: I stay outside nearly the entire time. Since they live in a warm climate, it is fine. I also plan on excursions to keep me occupied away from the home. Are there side trips you can plan for the kids sake such as a tourist activity? You could fit that in and even add in driving time as time to be away. If your mom and sister want to come along, please have them drive separately so that you are not trapped in the car with them for those 'talks' which inevitably come about. For me, I also had to be careful of the late nights when my uBPDm wanted to talk. I would get trapped in obligation and listen and listen as she talked and talked and always headed into blame... .I imagine you know how that goes.
How old are your children? What have you told them about grandma and their aunt? I opted to not say much and let them make up their own minds about grandma, but I was always there to watch out for them and their safety. Are your family members likely to show their BPD behavior around your children?
You can plan in those visits with your grandma without taking your mom and sis along, and perhaps you can add in a visit to a place your children would enjoy, without sharing the extra stop information with your mom and sis. Boundaries are the key here. They will be for you, and not for those who wish for you to not set them. Plan ahead as much as possible (which you are doing!

) and that can help a whole lot!
Wools