I have, he wasn't my closest friend of all time, but we were, and still are close to a degree....I was 'in love' with her, as much as I could have been at that age, and it was his older sister, he knew...We even lived in the same house, it was 5 bedrooms or so though, but it was uncomfortable at times, coming out of his sisters bedroom in the morning and seeing him, or all the drunken arguments and fallouts, us on the washing machine....
I'm currently going through similar feelings with another close friends sister, who's also older, I asked myself one time what the fukk my problem is...my intentions are always sincere though..I always genuinely like them and would be in a relationship, or even more with them, I'm not some grimy creep looking for a one night stand with someones fam...
I also asked for permission in the first case, before even kissing her, I dodged her first embrace of me, she was upset and angry, just stormed off before I could say anything, I just went to her brother and told him what was up, he knew we were feeling each other...She really respected that later, I think he did too...