Is it okay to be “antisocial”?

Honestly, it sounds like you're just too stressed and busy to be worrying about friends.. now i'm not here to say that you are or aren't antisocial.. but taking a break from people and having some "me" time might help you sort out everything you're dealing with. It not gonna be like this forever. But don't let it get to you. If you need someone to talk to.. talk to your sister or do what you're doing now. because all the people wanna do on here is support you. so keep your head up. you've got lots of support. and its not necessarily unhealthy to not talk to people.. it gets unhealthy when you start to allow that to affect who you are and how you feel about yourself. So as long as you understand that you're never alone.. you'll be fine. i'm just a message away as well & i'm almost always on the computer lol. keep your head up, love. (:

Sounds like you aren't antisocial at all, just too busy to maintain your friendships. It's perfectly normal and healthy to be an introvert, and I think everyone should be their own best friend. But you clearly are in a situation where you've sacrificed something important for the sake of someone else. It's wonderful that you're there for your sister, but you are your own person with your own life and it's very important that you look out for yourself, too! She may not even realize what you've given up for her. I suggest when you're ready, calmly express to her how you're feeling. I'm positive she will understand. You might even want to contact the friends you've lost touch with and explain that to them, too. It might be awkward or uncomfortable at first, but I think you'll be surprised at the results and you can end up working out a good balance of friends and family! Good luck, hugs!

Well it seems like you are too busy to have enough time to spend lots of time with your friends. That is not being anitsocail that is just life. And now to answer your real question, I belive it is. It more or less depends on you as a person and just what your style is. As long as you are ok with being antisocail or in some people cases', very social. If you believe it is the right choice and you are happy with it.

"Is it okay to be 'antisocial'?"
Yes.

"So is it okay to just stay like this?"
Not for you, since you're insecure about it. So you're probably not antisocial, or at least it doesn't make you happy in some way.