Best friends' sister

Hello everyone,

*dusts off cobwebs*

I have developed feelings for one of my best friend's sister. We have known each other for many years (10+), though in more recent years we haven't seen a lot of each other. I've always had a bit of a crush on her; She's a year older than me, and she has told me that she thinks I'm hot.

We decided to meet for a catch up at the end of last month (2 weeks ago), and she told me that she was surprised that we hadn't 'hooked up' before. We made out a few times before I dropped her home, but since then I've not been able to stop thinking about her. Her intention was to have casual sex some time in the future, but upon her mentioning that, it bought back all these feelings of raw emotion which I'd developed for her years ago.

At the time, I freaked out a little bit as I wasn't prepared for that coming out of her mouth. I feel I've worried her by raising my concern about her brother finding out, and by asking her why she dated a particular people at college. Most of the things she was asking me, I felt difficulty in answering.

We met a few days after this to go and see the fireworks, which ended up being cancelled due to rain. We still took a walk down to the beach and had a bit of a chat, I was really nervous and again had trouble talking to her as I was so nervous.

She has told me that she doesn't want a relationship with anybody just yet, and I'm not ready for one either. I just want advice on how to handle this situation so I can turn it into a relationship in the future.

I've thought about writing her an email containing all of my thoughts about her, but I don't want to freak her out or jeopardize the relationship I have with her brother.

I need advice on the next step to take. It's been really difficult, but I haven't contacted her for over a week, as I don't want to give out creepy vibes. For the record, I last had sex a few months ago (with someone else), and I do sometimes struggle talking to people who I find attractive.