If your friend is okay with it and if the sister seems interested, you may decide to ask the sister out. Make an excuse to hang out with her in a totally platonic setting. You may feel intensely for just a few weeks about this person before your feelings begin to mellow out. Have your friend come over to your house instead, talk to them on the phone, or hang out somewhere else. Or does she just rustle your proverbial jimmy? Life is short Glory is eternal. Usually, the first date is a casing-out process.
He might get upset or someshit, but atleast you've conveyed a subtle approach that makes him think rather than go apeshit crazy at the mention of you, crush, sister.
10 Steps To Dating Your Friend's Sister
But there are some nasty possibilities here. Just can't be bothered with hassle and it's far too close with social circle. You are a dead man. Losing them both in 6 months. The advantage of the dreamer is that he never has to face the chains of reality.