One of our rules in that situation (I was living with housemates for a long time) is the moment we move in we talk about things like that. If someone develops a feeling to another housemate we should talk. If things explodes it won't be good.
You can't control their feelings and you have to respect them, on the other hand if you can't handle them you should leave. They should respect that too. Talk to both of them, maybe even separately. If you have a good relationship things should work fine.
I also started developing feelings towards a roommate but when we talked we just realized that we won't work as a couple and the best thing is just being good friends.
Expressing feelings is hard, so maybe it is better to talk with her in person or even write her if you don't feel you can talk about it. As for the guy, tell him you like the girl too and explain how you feel about them acting like a couple... do it with respect. Maybe ask him if he likes the girl too... but don't show that you are in his way, just show that you don't want things to get messy and want to resolve a potential problem before it becomes a problem... avoiding problems I mean.
Finding a hobby is a good distraction, but it only works when you are not at home, it won't resolve the problems at home, and not being at home is ruining the whole concept of home...so you will still need to resolve the problem at home and not run away from it.
I hope all works out well.