we are just about ready to close on a house and the seller's ex wife decides to put a stop on the closing. She doesn't even own the house, it's under his mothers name but it was part of their divorce settlement. She claims that the house was sold for 150k less than the value but we got 2 appraisal's that suggest the house is being sold at a fair price.

Can I sue her for this?

Sue her for what? Your suit would be against the husband. If the house is indeed part of the divorce settlement, he knows it, and is the one defrauding you. But if the house is really in his mother's name, he can't just divvy it up with all the marital assets, so something is very wrong here. Two very glaring problems that shouldn't have squeaked by. The real estate broker, if there is one, is also liable for not knowing who the owner of record is.

That is if this is the real situation. Start with city call deeds office or assessor's office and find out who the owner is, and then demand an explanation of the rest of the story.

Perhaps you mean that the divorce decree includes selling the house, and the ex is unhappy with the price, not the sale itself. That doesn't change a lot, and you can hold everyone accountable, but it might not get you much without a civil suit, and you can't do anything about the delayed closing meanwhile. Sorry this is happening to you.