I think that your mother's reaction was quite normal. Just remember that in her day, homosexuality was illegal etc. Like Dirha said, it's a bad time of year. Also it seems that she had no idea that you might be gay. Whereas your sister might've had an inkling. So your sister may be annoyed that you didn't confide in her first...? If you think this is the case and her attitude continues, then have this out with her. Say that this is a difficult time for you as well and that you'd appreciate her support. Being of your own age-group, she is more likely to accept it sooner than your mother. But if your mother sees that your sister is ok with it, then that will help her to come to terms with it. So, I'd advise to get your sister on your side, even though it might entail a heated argument with her in order to clear the air.
Happyhead: I know you may not feel like it but remain calm and upbeat because your mother will read more into that than anything else.
I think that's good advice. It's easier said than done, maybe. But if you show that you are not ashamed of who you are, and are not upset, but relieved that you're out, then when your Mum and sister see that you are strong, they will come to terms with it more easily, rather than thinking it's a "problem"(they'll then try to "solve" it and be in denial!) or "a phase" that you're going through.
Get back to this thread Sheeben, and if you'd like to say how things are going, good or bad.. it's always good to talk.
PS When I was 21 and still had a bedroom at home, I had little postcards and pinups(tasteful, mind!) of the music/film stars etc. guys that I kinda fancied. Now that your out, think about what pin-ups your going to stick on your wardrode etc... just a thought!