I get on fairly well with MIL, DH and I have been married for 10 years and have one DS who is five. DS is MIL's only grandchild.
My AIBU is that whenever MIL arranges an event (birthday meal etc) me and DS are never invited, only DH. DH is an only child so usually it's MIL, her sister and DH.
I find it more annoying that she doesn't include her grandson than me to be honest! AIBU?