I joined this thread late and have not read every post, so I apologize in advance if this has been said already.
Firstly, her finances are not your concern. Her statements that she expects you to pay half her debts once you are married are telling. The next sentence about being a stay at home mom, tells me that in reality she intended to get married, not work and stick you with ALL her debts, and no doubt run up more. Her debts are hers... and if it were me, I would tell her to go to whomever she was messing around with for help paying if she needs it.
Secondly, she cheated on her last b/f. She claims that she did it because it was an abusive relationship and she wanted out. That is so illogical that it defies belief. Cheating is wrong. There is no excuse... EVER. If there is a problem in the relationship... and I dont care if it is boredom, abuse, or sexual dissatisfaction... cheating is still wrong. The thing to do it to deal with the issues at hand upfront and honestly, then break up or work it out. Cheating is inexcusable, end of story.
As far as her family telling you what is going on... well Kudos to them. They probably figure that you are a good kid and one who deserves better than to be trapped by a manipulative, cheating b!tch and put their blood ties and family relationship on the line to do the right thing. They offered her every opportunity to tell you and she lied. They must respect you and hope that by exposing her deeds, she will try to straighten out. Whether or not she actually had sex with another guy, well there is no smoke without fire, and even if she were just heavily 'petting', that is still being unfaithful to you. Unless of course you dont mind if she does that, but somehow I doubt that.
Lastly, where you go from here... She must give you the ring back. She broke your engagement when she lied to you. Get your ring back, sell it and use the money for a mini-vacation to get this out of your system. If you take her back, you are a fool and deserve whatever you get, but somehow I think you are above this. Don't lower yourself to be with a liar, a cheat and a sneak... you will hate yourself in the long run.
Cut the ties, move on and find a woman who will appreciate you.
Good Luck!