Bisexuality Support Group
Bisexuality is a sexual orientation which refers to the aesthetic, romantic, and/or sexual attraction of individuals to other individuals of both their own and the opposite gender or sex. Most bisexuals are not equally attracted to men and women, and may even shift between states of finding either sex exclusively attractive over the course of time.
Your sister is right, you can't have it both ways, and if you want a transgender lover you must look for new friends if they are too narrow-minded to accept you. You probably know that the transgender people are the courageous of the LGBT crowd, because their everyday is an uphill battle, So it is up to you to show courage as well. Good luck!