Like not going on trips until you learn not to be a bully?

We get it. You want to honor your tradition & make up for a terrible loss. However, you aren't putting what is best for your niece in mind now. Instead, you are focused on rewarding her bullying (yes, this trip is telling her that you approve of her bullying your stepniece (& others) which your own attitude seems to confirm as well in your comments) by taking her on a trip that can be postponed until she gets real help for the reasons she's being a bully. (Therapy is a good start.)

Also, the comment about taking your stepniece on a trip after her mom dies was cruel & shows you & your niece are very much alike given that's exactly how a mean spirited bully speaks.

YTA & need to realize that your niece isn't a victim nor innocent. You can do whatever you want, but don't you dare pretend that you aren't condoning her behavior while assisting in the bullying by doing what you are. She isn't going to learn that she's wrong when her role model who treats her special isn't doing the hard thing & allowing her to get what she wants while continuing her behavior.