It really is okay to date your friend’s sister, as long as you handle the situation well. Most guys are very possessive of their sisters and if you’re the type who likes to flirt and if this friend of yours knows about it, then you’re a dead duck! Obviously you will have to be a bit manipulative to keep calm and peace between the friend and the sister.
The first thing you need to know is that no matter how flirty they are, guys would always want their sisters to settle down with guys who have never had a girl in their lives. As impractical as it may seem, it is the truth. So her chances will initially be slim as I’m sure she doesn’t belong in that group. Don’t tell your friend that you’re in love with his sister. If you’re flirting with other girls, in the meantime, don’t tell him. Start doing something nice for the friend. Remember that the friend knows you well, so please don’t do a U-turn and become overtly nice. He gradually changes and stops talking about girls as much as possible. Tell your friend that your interest has shifted from girls to something else, something that is much more serious and mature.
Make yourself a little boring and uninteresting, but not stupid. Spend time with your friend as much as possible and do something for him that makes him owe you. Start meeting his sister in front of him, never allowing him the feeling that you like him. Treat her in a casual but friendly way. Don’t flirt with her when her brother is not around her. She should be able to trust you. Once the groundwork for her has been laid and you’ve seen each other a few times, you can ask her out of it, but of course with your friend’s permission. He may or may not grant you permission; this will depend entirely on how you present it. Tell him you’re only taking him to lunch. This is a very safe option, as your friend will know that you cannot have physical contact with the girl in broad daylight at a restaurant. He convinces the girl to go out with you too.
Once you manage to buy her lunch, keep repeating the process until the brother relaxes. At this point, I’m assuming you’ve charmed the girl enough to want to go out with you. Next, move on to the movies, this is a bit more dangerous situation for the brother, since once the lights go out, anything can happen between the two of you. Once you’ve repeated this process, take her out to dinner. But he always remembers to leave her at home after that.
It may sound tedious, but this is the best way to handle a brother. At the end of, say, two months, you will not only have a girl to love and sleep with, but also a brother and a friend.