MIL is just in denial that her son is messed up. He's weird and she's weird too.
U know my sitch. MIL & FIL are enablers of the childish behavior.
You don't have to ignore your anniversary. Celebrate that it was a part of your life that produced a great S.
My anniversary is coming up too - 21 yrs it would've been but I won't celebrate it with H - I'll just celebrate a part of my life that I totally committed to.