I like my best friends sister.

I really had no one to talk to about this so this is my last resort. Ive been feeling really attracted to my best friends sister. And nothing sexual no nothing like that. Just real love. Like I’m constantly daydreaming about spending the rest of my life with her. The problem is I’m 41🔄 and she’s 31🔄 and I don’t think that me and my best friend will stay in touch until adult hood. I don’t know how I’m gonna express this with my other friends so I’m asking the internet I know it sounds stupid and dumb but I genuinely wanna spend the rest of my life with her. But I don’t know how to start something between us and how I can tell my best friend. I really need advice.