Advice?

Hello ladies, so recently I lost my mother there has this one person, I’ve known him for about 4 years. I have always connected with him he’s always been there when I needed him, always knows what to say, he has really helped me with my grieving process... I find myself to be getting feelings for him, only trick is that he was my sisters high school sophomore year boyfriend. My sister is currently in a long term relationship and is almost 21 she “dated” my friend for 9 months and did not treat him very good during those months, now she knows I interact with him and she says she doesn’t care I can do what I want although she completely hates him. Am I a bad person for starting to get feelings for him?