Would you go...

My SIL birthday is today, fiance told be a couple of days ago that she'll be doing a small get together in the afternoon and asked if his mother already called me about it, I said no.

So this morning MIL calls to ask if I like to go to "Jessy's" birthday get together this afternoon (4 hrs prior) I said I already have my day planned, which I do (though I could squeeze a couple of hours in). I just wonder why my MIL has to call me last minute when Jessy could call/message me quickly herself and since this was planned days ahead. Maybe I'm being petty but I just find it ridiculous that I get invited by others on her behalf!

If you were in my position would you still go?