I really like someone. What should I do?

I know this is weird for some people to read but its perfectly normal to me. There’s this girl that I like, and I’ve liked her for a while now, but I don’t know if she knows yet or not. I want to tell her how I feel but I don’t know if she’ll feel the same way about me. About a month or two ago I was convinced I was in love with her cause even though it had only been 3 weeks without me seeing her, I had already felt like I was going out of my mind. But once I saw her again I was immediately happy and was happy for about a week after. It’s now been 2 weeks that we’ve been apart without talking and I already really miss her and want to see her again. When I’m around her I feel like I can be myself and relax and just be who I really am with no worries and no problems. I feel complete with her in my life, and I don’t ever want her to leave, and I definitely don’t want to lose her. But at the same time, I don’t want to tell her how I feel and for her to get freaked out and for it to damage our friendship. I did tell her sister about how I feel about her over the summer last year and she thinks it’s cute and she promised me not to say anything to her until I was ready, and I think I’m ready now. But it would help to get some outside advice first to see if others think I’m ready too. So please help me if you’ve been in the same position before, or know someone who has, or even if you think you could help it would be nice to get your opinion. Thank you for any help and advice you may give me :)