me personally.

I'd bang my mates sister if I was drunk.

but wouldn't if I was sober.

this doesn't make either of these options right.

If you wanna date her speak to him about it first.

I think it depends who means more to you your mate or his sister.
you have to chose one or the other cos if it all goes wrong you may lose your mate.

I personally steer clear (when sober) cos I think relationships are complicated enough without added tension.