You should start by practicing on how you are going to tell your sister-in-law that you love her to a mirror for about 10 times. You can tell her that you appreciate who she is as a person and that she is your role model and that is why you are fond of her.
Don't..... I mean that's just going to make matters worst then she won't feel comfortable around u. It's better if u guys just keep the relationship as it is. Would u rather have her comfortable around u and not know your feeling or her uncomfortable around u and know your feelings. U have to think things over