I'm secretly in love with my wife's sister
You really are in love with her?
It's probably all in my head, but I think I'm in love with her too much
It is quite dishonest to be having an emotional affair, especially with your wife's sister.
I masturbate to her and I'm too good to tell her this, she will reject me if I ever told her.
Well, I think that you are right to keep this to yourself, as nothing good would come out of revealing your feelings on this matter.  However, you should concentrate on the real love you have for your wife, not the lust that you feel for her sister.  It's likely that you are imagining your sister in law quite differently than she really is.