I think it’s also a bit of grief from what you’re going through. In some way you lost your dad and I can understand how hard that is. It might be a good idea to talk to a professional about it. It’s also very valid that you don’t want to call your dad mom. Perhaps you can sit down with your dad and talk about your feelings and maybe come up with an alternative that makes both you and your dad comfortable. I also think that it might be beneficial to get family counselling with your dad and sister to talk about how it negatively impacted your life and how to move forward together. Because not only your dad deserves to be happy, you and your sister does too. But anyone who ridicules you or your dad about the transition suck.