How do i arrange a romantic vacation with my sister in Spain?

My sister and I have a lot in common. If it was possible I would consider marrying her; However, I'm unsure her feelings are mutual; I'm unsure that she shares my romantic feelings. I'm considering a romantic vacation with her to Spain. If our feelings for each other aren't mutual, I'll definitely understand, for I truly have respect for my sister's wishes. We visited on a family vacation as children, but I would like to go again. How can I arrange this without being misleading.

Anyone out there been in a similar situation? What should I do? Constructive answers only please. refrain from simply displaying personal disgust. I know this seems unusual to some people but this is what I'm seriously considering.