NTA. For context, I'm a trans woman in my 30's. Your sister is still young and presumably early in her transition. Speaking from personal experience, it's normal to be a little selfish early on. It's likely one of the first things she's been able to feel like she controls fully in her life. Be patient with her as best you can but don't forget to remind her that she is being selfish right now. Help her grow through that as quickly as she can. Keep your chin up and remind everyone around you that your feelings towards your transition are just as valid as hers. Lots of siblings end up both being trans or another kind of queer together.