28 [M4F] Austria - my GFs younger sister

I’ve been in a long distance relationship for almost 4 yrs now. We usually see each other 3-4 times a year. Pretty challenging at times. Last summer we went to a wedding of a friend of her family, so her sis attended as well. I got to stay with the family. And got to witness my gf and her sis trying on their dresses. No nudity or anything, but I can't forget the sis. Since then l've been increasingly thinking about her. I feel bad about it, but it's exciting too somehow