Honestly this could be pressure from her soon-to-be husband. His extreme guilt over his gay feelings mixed with his envy that he has toward you might make him really hate you. He is gettin a hetero marriage that he never wanted while you are with the partner that he secretly wishes he could have. The most vocal and nasty anti-gay people I have met seem to be closted gays.
In the end forgive him for his inability to accept himself and forgive your sister for being too weak to stand up for you.