Reply: My finances sister

So im engaged to an amazing guy who I love so much he has a younger sister who just turned 18 she's beautiful she's funny she's real I completely adore her.we're kinda close actually pretty close but lately I've been feeling like im falling for her. better yet lusting after her im straight never been bi curious or anything but I think about holding her loving her cause she has no boyfriend. she's flawless and she complains about being lonely so I take her out a lot do things with her. Lately I can't seem to get her off my mind from thinking about holding her playing with her hair to straight up eating her out or shoving her face into mine. I've never once thought about another woman this way. I find it nasty but not with her she's so hypnotizing. I feel bad for my fiance but ugh I wanna make a move on her so bad I even think about her when I masturbate. i have seen her naked when she got changed one day and her body is perfect