Should i let it happen?

I'm 26, my sister is 22. This all started during our family vacation last summer, I accidentally walked in on my sister changing into her bikini. She didn't notice me at first, so I got a good look of her bra less body. I remember thinking to myself how physically attractive my sister was. Both her and I were sort of blessed with our good looks. We never lacked attention from the opposite sex and were both satisfied sexually well at least I know I was and we both experimented a lot of things, as far as sex goes, not with each other. So anyway, she finally notices me and freaks. She gets mad at first, yelling and telling me I needed to knock. I felt guilty for starring at her so I told her I was admiring her body. Then I showered her with compliments, she calmed down a little and told me she needed to finish changing and that I had to turn around if I wanted to stay in the room and vent. I told her that it was nothing I've never seen before and besides I already seen most of her already. She told me that only if I got completely naked too, so she doesn't feel singled-out. This is wrong, but her body turned me on and I did tell her that. She told me I wasn't bad myself either. Afterward we got dressed and walked down to the beach. We never spoke of the incident again.
Ever since then though, her behavior towards me changed. If she knew I were coming over to my parents for dinner or something, she'd walk around me just in her bra and panties. She'd model off her underwear to me, explaining that she needed a "guy word", some of those bra's were see-through. I never got the courage to straight up ask her though. One night she came home really drunk, while I was helping her get to bed, she kept trying to grab my d*** and asking me if I wanted a blowjob. Well at first I thought she was just drunk and wasn't realizing that I was her brother, but then she said something and threw in "your little sister". Nothing happened though, I put her to bed and she went to sleep. The next day I told her what happened and she pretty much told me that she's been sexually attracted to me ever since what happened over the summer. She told me that she wanted to participate in one incestuous act for a long time and she wanted to with me, but she couldn't make herself because she didn't see me that way at all until I guess that summer day.
I'm supposed to drive down to my sister's college this week-end, pick her up and take her home for break. In a text message, she asked me if we had to drive back right away or we had some time, because she had a surprise for me. I kept bugging her with what it was, but instead she sent me a picture of her vagina, with "that's the surprise!". Should I let it happen? I don't want sex, would she be ok to just perform oral? We are both adults, and we are both ok with it. I still have doubts though. I don't want to lose my sister. I don't want to upset her nor do I want whatever happens this Friday to haunt her. What should I do?