I say 'trap her'; get your brother to leave some of his money inconspicuously lying around, baiting her to steal it without knowing it's his, and see how they both react when she ends up stealing some of her own boyfriends money!
Helping your sibling and their new family is difficult alone, and when your sister-n-law or brother-n-law disrespects you and your sibling, it just makes matter worse. Especially, when it affects you and your parents. Sadly, there's not much you can do except watch it happen and take it's own course. But trust me, when a liar constantly lies about shit, they will screw up enough to where no one around them will believe them.