I think I would be as loving and as protective as Buffy seemed to be, especially if our parents were dead and it was up to me to raise her. Of course, my sister is 21 and has her own apartment and a job, so the reveal would have to have occurred several years ago for the whole "raising her" thing to be relevant. I'd still want to prevent the Goddess of bad home perms from twirling her around in a Hell lock, though. And I might even die to protect her. She's my sister. Plain and simple. It wouldn't matter how she got here. What would matter is just that she did get here and that we *feel* like we grew up together.
When she was a kid, though, I sorta hated her so... I think I would have let Glory get a few cuts in before I saved the day. Or maybe I would have shoved her into the portal instead of taking the swan dive, myself. I know that sounds cruel. And yeah, it kind of is. But what can I say? Sibling rivalry is a bitca. My sister and I had to grow up and move to opposite coasts before we ever started to feel close to one another.