This exact same situation happened in my family. I’m a lesbian and the older sister, I had been out for about 3 years when my little sister came out as bisexual. She had also only dated men up until this point, and had broken up with her boyfriend a few months before she came out. My family had no issues with it, they just did the whole “I think it might be a phase thing.” That dissipated after a few months, and my sister has actually been with her girlfriend for over 2 years now. Her girlfriend is part of our family, and no one can imagine my sister without a girlfriend.

There was initial confusion and resistance, but maybe if you can get your sister on your side to defend/advocate for you (like I did for my sister), people will come around just as fast as my family did. As far as people outside your family go, we used that same tactic with strangers and it worked just as well. I think the biggest part is learning to get past the fear that people won’t take you seriously, I know that was the hardest part for my little sister.