Am I the asshole for ruining my daughter's school trip?

My daughter currently isn't talking to me because of this. My daughter's band is going on a trip to six flags in March. They needed to pay $200 each to go. It's due by the end of December so they can get a count for all the kids. How her school was doing it was the band kids had two options on how to pay. Option 1: Just pay the $200. Option 2: Sell these little discount cards to pay. The discount cards were $20 a piece so kids would have to sell 10 of them. My daughter chose the discount cards and she spent 2 weeks going to rich neighborhoods to try to get people to buy them. She sold them, and paid her band teacher so her spot for the trip is saved. My sister who I haven't seen in 5 years reached out to me to ask if I wanted to come with her on a trip in March. She will be back in town. Her wife is in the military so I NEVER get to see her. The trip was gonna fall on my daughter's trip. It wouldn't be a problem but my 14 year old son can't stay home by himself. He's on the spectrum and gets anxious and he just can't be home alone. I tried to find another time to hang with my sister and it wouldn't be for possibly another year. So I tried to talk to my daughter. See if she tells them she can't go if she went with option 1 they would refund her the money back but since she went with option 2 the school gets to keep the money because it was their discount cards they gave her. So all they really lose is time. I apologized to her and told her this is my only time I will be able to see my sister and she needs to be the big sister right now while I'm gone. She obviously didn't take it well. I feel bad and she's pissed off and saying the typical "I'm moving out when I'm 18" stuff. I understand I should not be making plans when plans where already made but this is my only chance to see my sister if I don't want to wait a whole year. I wish I could compromise but I couldn't think of one that would work in both our favors.

Edit: I can't bring my son with me. The point of the trip is my sister wants to get away and relax for once. It doesn't make sense to bring an anxious autistic teenager on this trip. It would be ruining her trip.