You said things have been rocky the past year, but this work with at risk kids has to have gone on for a long time if one of those kids is now a musician in residence, so is there an issue about you perhaps feeling like your mom invested in these kids more than with you as her daughter back when?
Other than that, I'd go with the pp who wrote:
If parents knew OP turned down SIL’s birthday celebration in order to spend time with them and THEN announced they were going to the performance, that’s annoying.
If parents didn’t know that OP had cleared the calendar to spend time together, then it can still be annoying to OP, but she should chalk it up to a communication failure by both parties.
In neither scenario is this a hill to die on.