MIL regularly misses DS1's birthday party (he has had 3, 4th is coming up soon, she's been to one). She has not missed any of DS2's (just turned 2).
I realize that she is INVITED not summoned, BUT DS1 is getting smart, he notices these types of things.
She organizes a women's retreat and always schedules it for his birthday weekend (I pick either Sat or Sun depending on when DH works). I always pick the closest weekend so I don't have to choose between people to accommodate. My dad has a swing schedule, FIL has work as well (so far they have both found ways to be there regardless).
What should I do? Let it be? Inform her she isn't invited to my other two sons parties that year if she doesn't come to his? I don't know, but my sweet boy will be so hurt if she doesn't come again, he still remembers last year and that she came to his brothers in Dec.