Should I tell my best friend little sister that I have feeling for her?

For some context I(21m) have been best friends with Harry(fake name 21m) since elementary school . We grew up together and have been super close since high school. I am super close to his family to where I have been on multiple trips with them and they call me their second son. Me and his is little sister (19f) didn't really talk until around a year ago. This past year however her n I have spend a lot of time together with their family and some one on one time. To make this clear I never had any intention of having these feelings for her but one day (around 11 months ago)I started to notice her more and more and seeing her more than just my best friend's sister. She is very sweet, kind and caring person. I never planned on telling anyone about my feelings for her but I ended telling Harry and he told me that he wouldn't mind if something happened between me and his sister. But I am very scared that his family especially his parents would not be very accepting or supportive of my feelings for her. Now I have no idea if she feels the same way as I do but a couple months ago we went on a trip and one night while we're hanging out I put my arm around her and she leaned into to and put her head on my shoulder. I know that does not sound like much but it really meant a lot to me. After that night I noticed that we would always find ourselves sitting together or her touching my arm or shoulder. Now their family is big on physical touch so I do not want to over look these things but sometimes when we talk the way she looks at me it hard to believe that my feeling aren't reciprocated. I have been dealing with these feeling for way longer than I would like and I feel like I am at a tipping point where I can no longer continue this. Please offer any advice on whether I should i tell her and if so how should I tell her. Sorry for any spelling or grammar errors I typed this quickly.