Hi Rottiemom;
We've coped with the same issue with my husband's kids' mom (uBPD) and her husband (uNPD). My H and I got married when the kids were 5 & 7, so we have endured many years of going to sports practices and school performances and birthday parties, and on and on, where we knew that they would be there. It is so difficult to "amp yourself up" to go and focus on the kids knowing that there will be disordered family members there

Like Leaf56 mentioned:
leave and go to your car and sit to see if you can figure out an alternative way to watch, maybe reposition your car and watch from there and then meet up after the game so you can at least discuss the things you saw your grandchild do afterward.
Sometimes we have to "pre-game" some ideas for how to navigate these situations, before we get into them. It can help us feel safer and in control to decide ahead of time something like above: if I see her there, I know that I can go to my car, lock the doors, park my car on the other side of the field, and talk to the grandkids afterwards.
We had to do similar stuff. We are not able to be the "Hollywood stepfamilies" where we share Thanksgiving "for the kids' sake" and chat at soccer games. Things go best for the kids when we have a lot of distance between us and Mom/Stepdad. We cannot sit next to each other at events, so we have to think ahead of time of where to sit, whether they got there before us and are already sitting, if we have friends already there we can be next to, etc.
We also don't always try to chat with the kids right away after an event, unless it looks like it'll be simple and quick to catch them before they go to Mom. Sometimes we let them have that first moment after the game/event where they run to Mom, give it some time, then we go up afterwards.
Of course, in some situations, it does work OK to catch them "right away" so that they know you were there and proud of them, and then you can take off quickly so that you don't prolong your time at the event (and increase chances of running into the pwBPD).
Know that there are many options out there for you, and even if the options don't look "typical", well, we aren't in typical families, so it's OK!
Let us know how things went;
kells76