Trying to be creative:
Tell your brother that adding her to the deed is a violation of the due on sale clause and transfer restrictions of his mortgage. So, if he really wants her to have an ownership interest in the property, in order to "avoid liability" he should instead transfer the property to a newly formed LLC that will hold title to the property. Then, you can make bro and psycho-girl both members of the LLC, but when drafting the operating agreement, include a provision allowing him to buy her interests at very low price, and vest all power regarding the LLC in your bro - including the ability to sell the property and use the proceeds as he wishes. Perhaps they are not sophisticated enough to catch onto the terms of the operating agreement.