shopacholic, if you are ready to leave, this may be the perfect time. if your family is looking at you as some kind of pillar, you may be able to help them. they know you are moral, not addicted to drugs, not looking to be a wild college kid, etc. if you choose your words correctly, you may open a few eyes. maybe, you can talk to your niece and sister-in-law and form a sort of battle plan. do you trust them?
i would talk to the niece, then, when you talk to the rest of the family, raise your own objections, and make it like they are hers. maybe, if you tell her your side, explain what's going on, and tell her your objections, she will go along with it. then, you can tell the family, this is what she's saying, and she's making sense, get them to research it, and act surprised by the results.
i didn't explain myself very well, here's an example: after talking to your niece, and getting permission, have a conversation like this:
you: i talked to the niece, she has some valid points. if we are going to get her back, maybe we should research it, to show her she is wrong.
family: well, what is she saying?
you: well,..........
after this point, go into any of your ideas, and get your family to research it, they may start thinking. after the "research" is done, so, whoa, wait a minute, she may be right about this!!!! and act all surprised.
just a thought.