I'm in love with a good friends sister
How did you fall in love with one of your good friends sisters?
She is just the perfect girl for me. She checks all of my boxes off.
If you feel that way about her then you should let her know. However, don't expect her to necessarily feel the same way.
It's fine. She'll be into it.
I hope she feels the same way about you. It sounds like it would make you feel really happy if the feeling is mutual. Being in love can be very pleasant!