@Dontknow-17,
You are transferring your closeness to your sister, in talking about your issues, to a crush.
I suggest finding a way to get therapy - and therapists are bound by physician-patient confidentiality; they can't tell your parents or anyone else what's discussed in a session, unless it involves you talking about wanting to hurt yourself or others - as you need to speak with someone about your OCD, etc. who is
not your sister.
I assuming you are properly diagnosed, so getting therapy should be a matter of going to your pediatrician and requesting a referral. And, you guessed it, there's physician-patient confidentiality there, too. Even though you're a minor.
Oh and one more thing - most seventeen-year-olds are pretty sexual. That's likely not, specifically, anything odd. It's the subject of your fixations that's bothering you, yes? But the fact that you're highly sexualized at that age is pretty damned normal.