We once had a party, and a mom dropped a child off and said something I didn't follow. Her daughter was having her party the next day. We had been invited, and I never got the invite (my daughter never handed it to me). I felt terrible. Had no clue.
Normally I am a neurotic RSVPing. It happens to the best of us.
She may be frazzled. She did thank you for getting back to her, so to me, she sounds like a polite person who is considerate? (she took the time to thank you for RSVPing).
Does your daughter want to go? You must have thought so since you accepted already.
To me, I'd go. I'd assume she's coming for now.
Don't be petty (which is easy when frustrated and reacting). Focus on daughter's day.
It is a pain when people don't RSVP. I get it. But move on.
I've had it happen when kids go to dad's house and the invite stays there, or when people just wait till last minute ..
If your daughter wants to go (it's about her), let her go. If all the kids are going - she'll want to go too.
Don't let it bug you. Move on. Not worth it.
It may have totally slipped her mind.