And you want to ask her out, and you want my help. This is a terrible situation. It might be less terrible if you carefully read this ten-step guide.
I know it very well. This can be inconvenient. And that becomes truer the older you get. Even if you could have a fantastic relationship with this girl, it might not be worth it, considering that such a relationship would certainly alter your friendship, and might kill it off. This is not my intent. And this can be hard to figure out, as a sufficient level of arousal can make every person of every gender think their potential lover is the most interesting person in the world.
So, is she actually cool? Or does she just rustle your proverbial jimmy? Men tend to overestimate the interest level of hot girls, because emotion is contagious. Often, this is not true, which is why the first date is a valuable no-stakes screening procedure. One date with this girl could have a huge effect on your friendship. So, you should do some preliminary examination first. Make an excuse to hang out with her in a totally platonic setting.
Do some sort of vaguely excusable professional coffee. Tell her you want to plan a surprise party with your brother. Just get two hours to chill out with her, and see whether your perceived chemistry is a real thing -- if you have the kind of bubbly, clicky conversation that we all dream about. If so, and if you still want to go ahead, then you have to take the painful next step. Tell Your Friend Yes, tell your friend that you plan to ask her sister out. But be aware that your friend might freak out, throw a drink in your face, put you in a triangle choke, and spend all night writing your name in his blood on the wall of his holding cell after he gets arrested.
Or he could be totally cool with it. But there are some nasty possibilities here. Ask Her Out Whew! Now, just ask her out, like you would any other woman. Which is to say, suggest alcohol at a time and place, without apology, hesitation, or further explanation.
Time to move on. But if you do: Just be honest, forthright, and remember that oral sex is your responsibility, too. By all means, engage her in discussion about it if she brings it up. The ideal of talking about everything all the time is an ideal promoted by idiots who have never been in a relationship. Be Discreet Know how you told all your buddies about how Michelle was a total freak who fulfilled all of your perverted schoolgirl fantasies? And this is an even worse habit in this situation.
Men in new love tend to disappear into their new romantic relationships and forget about their bros. This is an unhealthy thing to do in general, and doubly so in this case.