I know it's not as easy as it sounds probably, but I would leave her, too. I mean, you can try to talk to her first and see what she says and if she even feels anything for you. After that, though, granted that things don't work out, I would just leave her.
either tell her that you're not comfortable with her pretending and that she needs to man up and stand up to her siblings, or dump her so fast and hard that she regrets not coming clean and standing up to her siblings.
leave her ass!
Sounds like a cover for getting caught talking about her *real* other boyfriend.