I got married over 30 years ago and tbh although my sisters and my SIL were my bridesmaids, I didn't invite any of them to my hen - nor my mum or MIL because it wouldn't have been their thing! (and it was pretty tame compared to hens now).
Only you know the nature of your relationship OP and the nature of her relationship with the invitees.
It just occurred to me that I don't actually know whether my SIL had a hen do?! DH and I had been together for years before we got married and married for many years, with 3 children, before she got married. She demanded to be my bridesmaid before I had even had a chance to ask (or not). I would have asked if she had waited because she's my DH's only sister. I've known SIL since she was a teen at school.
So, fast forward, SIL getting married. There was a context of family illness, but it wasn't an excuse for what happened! DH was invited to the nuptials, and I wasn't! I was taken aback - we never had any fallout - and was even more annoyed because DH couldn't see any reason why his wife of, probably, more than 20 years at that point, wasn't included? Neither were the only 3 children to whom she is an aunt by blood. She never really did have much time for me or my children. He went to the full wedding. The kids and I went to the evening do but the weather was bad so we didn't stay long as it was a distance away.
Tbh I wasn't bothered. I wouldn't have enjoyed it anyway, barely knew anyone. I'm civil, but I don't go out of my way to make any effort. I'm not bothered any more. It is what it is.
My advice is, let it go. You are upset and she probably doesn't give a flying one. I'd be most p!ssed off that your MIL has clearly tried to keep it quiet that she's going. Wouldn't say anything about that one either, because no matter what, she's your DH's mother, and 'competition' is ill-advised!