I'm also a member of Alpha Phi Omega, a coed service fraternity. While we don't have a house, most of the members are very close and hang out a lot. We have definitely had brothers date each other, and several of those couples have broken up. No, it's not particularly pleasant, but it's not our place to dictate dating rules (with the possible exception of pledges...and not everyone agrees with me there).
Break-ups aren't easy, period. As much as you love your house, I think you have to step back and trust your sisters to make the best decision. But from the tone of your message, it sounds like you're also concerned about how you'll be perceived. I can't really offer much advice on this, except to not freak out over it. If you make it into a big deal, it will become a big deal. ALL of our organizations have gay/lesbian/bisexual members. There's a webpage,
www.lambda10.org, that has a lot of resources about being gay and in a GLO. Remember to love and support your sisters, and good luck to your chapter.