I used to post on Step Talk, very glad to see the forum is continuing in new form here.
I'm SM to 3 SKs in their 30s, all of whom were used extensively as pawns by BM in her besiegement of me and DH when we got together after their marriage had ended.  DH and I then had two children, now young adults too.
There were difficulties with the SKs and DH/me which were awful at the time but did get ironed out and I would say I'm actually close to two of the SKs, though everyone is super careful.  The third of them is now engaged to be married.
The fiancée's parents have now invited both families to the engagement celebration party that they are throwing for their child.  We can't believe they don't know the history, which is that BM is a narc and makes very public scenes at every opportunity, particularly in the presence of her children.  She is also dead set against me attending any family event.  To the point that the SKs as they grew older, were forced to retract their invitations to me to attend milestone events of their own because BM had ordered them to.  As a result, DH had decided he could not go either.  
If BM does this here once again, then the same will happen and I will have no issue.  But if she does not, quite simply because she cannot control these other parents like she can her children, there is a high risk she will just wait for now and then do something manipulative or confrontational at the gig.
My question is, in this situation would you just not go at all, or would you go and just try to manage any crisis that arose?  Normally I would simply not go at all just to avoid the grief, and leave DH to it, but DH naively told our own two kids that they were invited to this gig and I most certainly would not be leaving them to face any music alone.
Anyone had a similar experience?  Or any ideas?