I have this friend who can push my buttons some times. I've started to drift away lately but my sister is still relatively close to her. She was suppose to have a party two weeks ago but cancelled without saying anything until I asked. She sent out another save the date (as a text) for this weekend about a week or so ago. I saw it and acknowledged it but assumed that it was a casual thing she might cancel like the last time. Never head another thing about it. (she spoke to my sister about it a bit but she has a habit of not inviting me to things directly and just going through my sister like we're the same person). No date, time, nothing, just the save the date.
Today get a text saying somewhat passive aggressively "so I guess you guys aren't coming, because I haven't heard back from you". I responded back with what time? She says "whenever".

It's an hour and a half away, I don't want to show up and the party hasn't started. She finally says 8 to the next day.
My question is if someone sends you a save the date, is it up to you to follow up for times, etc?
background if interested, but not necessary for the question: I'm probably going to go because my sister wants to (she feels guilty) and would like the company and to be honest this friend has a habit of making fun my sister and I for being shy/reserved if we don't go to her parties. She'll get passive aggressive and mean to the point that we feel like we are lame and should go, but when I have something? She just won't come, "forgets about it", or comes for literally two seconds and leaves saying she has to go to the gym or out with someone else. Stuff like this is why I'm starting to drift away a bit.