The answer depends on several factors. Let's go through them. How did she approach you about it? Subtle or direct? The latter is usually a trap, but not always. How close are the sisters? If they rarely talk then it's less likely to be a trap. But you need to do some investigative work on that one. If your gf is trying to trick you you sometimes can weed it out of her with some casual inquiry about her sister. And finally, is the relationship you know, good? Is she performing adequately in bed and loving? If so, then don't cheat. It's just lame.