Does it matter? She's happy, he's happy, the wife is happy, right? Everyone's happy, except for the people outside the relationship. Guess what? The people outside the relationship don't get to have a say. She's an adult. She can do what she wants. 

You're passing judgement on her based upon your own morals and ideals, and you're not taking into account whether she's happy or not. You've decided for yourself that she's ruining this marriage or marriage is sacred. This is all *very* close to the same rhetoric that straight assholes use to try and get gay marriage made illegal again, so I mean you kind of *are* being a hypocrite. Whether or not it's a choice is irrelevant and a really piss poor argument against it honestly. But, the married couple have *also* made a choice to open their relationship up, it's their marriage, they can do what they like with it. Trying to say all marriage is monogamous and whatnot is *very* exclusive. Your sister hasn't forced herself on them. Maybe she's just not a monogamous person, which is *okay*. I am sure there are TONS of polyamorous people who identify as queer and LGBTQ+ who would absolutely tear you up for trying to divide them from the rest of the group, by the by. Go post this on r/polyamory, I bet they won't be nice. 

If she's not hurting anyone, and if she's not causing this couple pain, if this wife has given her blessing then who literally gives a shit? Except your family, I guess. Also you don't *need* to wrap your head around it, just like no one needs to wrap their head around your being attracted to the same sex. It doesn't need to be understood. If no one is being hurt, if people are happy, then it's *fine*. Accept her or lose her are pretty much your choices right now.