There are no impediments to your dating her, unless of course she says no! Best friend's sister could turn your best friend into your brother in law. So, the sales pitch isn't "I'm fucking your sister", its "I'm dating your sister and she's a wonderful woman." There is nothing at all wrong with that, its not only normal, it might be beautiful. And you have to keep up that line of discussion even if you're just fucking her for the sake of fucking her.
Honestly, it is unlikely your buddy would react badly to your doing his sister. It might be awkward, but its hardly a big drama. Go for it.