that’s because they are strait guys and any normal guy that does not have a kink in the area of actually wanting a transgendered boy as there girl friend wont want one.
I have thought about this issue a lot because one of my siblings is transgendered. if you are asking some one to except this you are actually asking them to go against there own personnel sexual feelings and identity. the truth is That being transgendered is a Mental illness and should be treated as one.
Sorry to hear that. We probably all have known of a family with a sibling who has this disorder. (I once knew of one such family.) I hope your sibling one day finds a way to recover under the right psychological care.