Sorry you're having such a rough time with your sister. I can understand how it must hurt to have a close relationship feel jeopardized.

As for overstepping your bounds...hmmm... If this didn't possibly ultimately affect your brother, then it is something that might be a little off limits. But it has the potential to affect your brother, so it is your business since you have his best interests to consider. I would, however, hope that even if the girlfriend found out, she would understand that it's not your brother's fault and that it would be out of line for him to be penalized for her boyfriend's and your sister's transgression. I do think that your sister needs to back off though, before making matters worse.

I wish I could offer you some sort of help about how to mend things with your sister. I don't have much of a relationship with my sisters, so I'm at a bit of a loss there.

I hope things work out well for you and your family.