AITJ for refusing to BABYSIT my sisters kids so i could go to a concert I waited for 6 months?

I (26F) have a sister Blessy (32F) who has two kids (5M and 3F). I love my nephew and niece, but my sister always assumes i am free to watch them because i am single and dont have kids of my own.

So here is the problem. I bought tickets to see my favorite artist in concert about six months ago. These tickets were expensive for me, like $300, and i have been so excited. I took the day off work and everything.

On the day of the concert, literally 2 hours before i need to leave, Blessy calls me. She sounds panicked and says her babysitter canceled last minute. She and her husband have a wedding to go to that night. She didnt even ask, she just said, i’m coming over to drop the kids off, thank god you are home.

I told her no. i told her i have this concert and i cannot miss it. She got really mad. She said It is just a concert, this is a family emergency. I told her a wedding is not an emergency. It is a party. If she cant find a sitter, one of them should stay home.

She started crying and said i am being selfish and that i care more about some stupid singer than my own family. She hung up and ended up missing the wedding because her husband went alone.

Now my mom is calling me saying i am the jerk bcoz blessy was looking forward to this night out for months and i ruined it. They are saying i could have sold the ticket or just skipped it to help my sister.

I feel bad she missed the wedding, but i dont think its fair that i have to give up my plans just because i dont have kids. AITJ?

TL;DR: My sister tried to drop her kids off at my house last minute so she could go to a wedding. I refused because i had expensive concert tickets i bought months ago. She missed the wedding and now my family says i am selfish.

AITJ