My best friend passed away last year and I’m in love with his sister

My best friend passed away last year from cancer and although he obviously wasn’t well , his death was still a shock because he wasn’t at the end stage of his life he was nowhere near it so it wasn’t like he was told he was going to die , he was still having treatment and bravely getting through it day by day. He collapsed at home he woke up one day saying he felt strange and then he collapsed , he went into hospital and passed away and it was so quick that still we are in shock. People on the outside might think well he had cancer why are you surprised he passed away but it really was a shock.

During his cancer treatment he obviously had days he felt very sick or his mental health would be quite low and the way his sister took care of him , it made me have even more respect and love for her. They were so close and I’ve known his sister a long time but became closer to her when my best friend/her brother was having treatment. After he passed away we’ve become closer , we are helping each other through grief and for a few months I convinced myself I am crazy for thinking I’m in love with her and I convinced myself that we were just bonded because of grief because we both are devastatedand I’m not actually in love but the more time has passed by I realised I am in love with her. I care for her deeply, I enjoy her company, she makes me a blush and smile, I get excited to see her and I think about her so I know my feelings for her are genuine. Sometimes when we spend time together the way she looks at me I think she loves me too but at the same time I feel it’s risky to tell her how I feel because I don’t want to look weird by expressing my love for her while she’s grieving and I don’t want to be disrespectful to her my to my best friend because I don’t know if it’s the right thing to date/ be in a relationship with his sister. I go back and forth over if I should tell her or not, she means the world to me and if I don’t tell her will I regret not taking that chance or should I never say anything?