AITA for missing my nieces first birthday

My sister's daughter is turning 1 Thursday. She's the first baby in the family so of course everyone is excited. I don't personally want biological kids, but I do foster children. I recently took in 3 boys. 15, 15 and 14. They were gonna stay with my husband while I went to my niece's birthday which was gonna be Saturday but my sister called everyone today changing the party to Thursday. That's gonna be a problem. I'm not going to put out all the boys' business but the caseworker told me they all need to be drug tested on Thursday and the 14 year old needs to be tested for STDs. My nieces party of 2-4 and their appointment is at 2:30 and where they are having the party is an hour away. I told my sister and she told me to ask the caseworker if we could reschedule. I did and she seemed annoyed. We couldn't reschedule for next week because this specific lab is closing the ENTIRE week of Christmas. I don't know why but they are and for some reason DHS wants us to take them to that lab specifically. She did try to see if we could schedule some other time this week and the only other time they have is Friday at 10am but the caseworker said that isn't going to work. The drug tests only take about an hour to get results on but some of the STD tests the 14 year old needs can't be tested there and they have to send it to their reference lab and they get the results back the following day but since it's a Friday and they are closed all next week we won't get the results back until over a week later and they need to know the results as soon as possible. I told my sister and she is really upset. She asked if my husband could take them and he can't. He's a surgical tech and they have a 6 hour surgery that day. She starts saying how she wants all the family there and I apologize and promise to send her a gift. She then starts asking why the boys need a drug test and why would a 14 year old need to be tested for a bunch of STDs. I told her it's really not her business. I don't even know everything but I'm not giving out their personal information. She says if I'm going to miss her daughter's party she deserves to know and I said no she doesn't. Then she starts asking why I even took them in to begin with because they seem like trouble. I told her if she's just gonna insult them this is no longer a conversation between two adults and I'm not having it. She called me a terrible sister and hung up. I guarantee I'm not the only one who's gonna miss my nieces birthday. She changed the party from a WEEKEND to a WEEKDAY. But I do feel awful. This is her first child. But the state wants them drug tested and stuff and when I agreed to take them that's me agreeing to take care of their medical needs. Still I feel bad and need to know if I'm being an asshole.

Edit: I know I gave too much information. That was my mistake. She was just already upset and I was hoping she would be understanding if she knew how important they were. That was my mistake.