Yes, I already did.
Some years ago, when I was 17, which is still young but at least a starting point in relationships. I frequently visited his house, but hardly saw his sister, who was quite shy. After half a year period where I've had it rough and didn't visit, I came once more and suddenly it hit off with his sister. Can't exactly tell what happened.
We dated for more than a year. I didn't have much time for him when I visited anymore, but the fact that I visited so much meant that we did more things together, so he wasn't really bothered. I think he didn't even care much, or at least never showed this to us. I eventually parted with her as friends, and since I saw her not as many times a before, it also meant I saw my friend less.
She's still the best girl that came into my life, so I have no actual regret for dating a friend's sister.