I think she needs to take part in the basketball game.
Just because she is on the "C" team doesn't mean she gets a pass to break her commitment. I guarantee you the "A" kids would not even consider ditching a game to go sit & watch a soccer game. It's like saying that because she is not a star player, it's okay for her skip out once in a while.
What kind of message are you sending her if you allow her to break a promise she made to several other people, just so she can go do something "fun" for herself & toss other people's feelings in the garbage?
I think this is where having your kid in too many activities backfires & puts too much pressure on them They want to be a part of both lifestyles and as a result, you have a 12 year old being pulled in 2 different directions needlessly. If she feels so strongly about watching a soccer game, then she needs to play soccer.