I'm really not comfortable with him being the ML because his current relationship is so go...

I'm really not comfortable with him being the ML because his current relationship is so good. I think it might be that the sister passes a way in the future rather than a cheating situation because thus far he doesn't seem like that kind of person.

I really hope its a coming of age like story where she comes starts developing interest in the opposite sex due to him, it would be that his presence in her life has basically set her expectations for what her future romantic endeavours would be like and by becoming aware of this fact, she becomes overly conscious of him hence there is some awkwardness which everyone notices and tries to give her time. Then as we go through the story, she comes to term with her miniature crush and ends it, experiencing an emotional growth and goes on to find love elsewhere.