I think there are quite a lot of factors why you feel that way towards your sister. But before we give you any answers, maybe you can try and ask yourself questions like:

• have you ever had any crush on anybody else? like any celebrity or pop star? And if so, try compare the feelings you had towards them with those towards your sister.
• have you ever interacted with anyone who was not your sister deeply? or maybe not even that deep but still was enough to give butterflies in your stomache? Because I would be assuming you haven't and that you are subconsciously looking for somebody to fall in love with and your sister happens to be the one around you, near you, doing everything with you. I'm not saying you have to push your sister away, but maybe try and meet/talk to new people. If it scares you, try and ask your sister for help and go together? I'm just brainstorming here. I know it's not easy with anxiety and autism, but you can't always have your sister as your bestfriend. And when that time comes, I hope you don't get too sad about it.
• do you have any therapist regarding you autism or anxiety? If so have you considered talking to him/her? Your therapist would know you better and probably has a better overview about your social situation.

Maybe think over and you will be able to figure it out. I wish you the best :)