Two friends of the opposite sex have a good relationship and become brother and sister, but because my brother doesn't cherish it, my sister is very sad … how to apologize?

Tell your sister what you like very much and why she doesn't cherish it, because she really can't help it. Since you are brother and sister, you should talk about everything like a family and say whatever you want, because you are brother and sister and a family. I also want to tell my sister that since my brother has admitted his mistake, you should give him a chance to turn over a new leaf, so that you will be happy and your brother will be happy. Do you want to never know your brother again? It's fate that you met, right? Think about why we didn't meet this time. I think your brother knows. If you are wrong, he will be sad, too. Why not make yourself feel better? I hope you can still love each other like real brothers and sisters.