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I wouldn't read to much into your friends reaction. I got a lot of pretty disintrested respinses when I came out to friends. I'd tell them I'm bi and they would be like "Really? Good for you. What do you think about that weather we've been having?" It's important to remember that while comming out to someone is a big deal for us, it's often not a big deal for those we tell. As long as your friend is supportive of you, that's what's important. She may supprise you in future too, by being much more understanding then her initial appathy would suggest.
As for the sister, I guess it depends on what she meant by "strange." if she meant it as "that's something unusuall that I've never heard of before" I wouldn't be to worried. However if she meant "strange" as in "wierdo or freek" then that's really unfortunate. As long as you don't think she'll blab to anyone you don't want to know I doubt it will be a real issue. Just remember she isn't someone to confide in in future.
As for fealing discouraged, I would recomend you try to stay strong. Don't let this one bad incident keep you huddling in the closet. If you feel you need to come out, then do it. Perhaps next time it might be good to ask people about thier views on LGBTQ+ stuff, specifically asexuality, first? That can be kind of hard, especially with something so specific and rather rare, but it could help you gauge thier responses ahead of time.

 

Don't worry about it. Just be yourself and the people that support you will be there for you. You'll get mixed reactions, probably positive ones for the most part. I've said this in other posts but it's a good quote. " what other people think about you is none of your business " good luck and stay strong!