couple questions....
i have a few ideas for you, but need some info. need general idea where house is located (just say a town). how many bedrooms, what are the property taxes?
Genius is on to a good idea....if you cant afford to buy them out, but the RE agent is saying it is worth nearly $500k, let the agent try to sell it....set the contract at some ridiculous price (say $550k). RE agent wont get that, but if you see bids come in reotely close to $500k you can pocket some decent money if you get that price. you will have to accept/reject any offer, and if the offers come in lower than the contracted price, you have no obligation to sell. If you are correct about the value of the house, you will either get zero bids, or they will be closer to $400k. Either way, dont play it so you are openly adversarial with your douchebag cousins.....when they see it is not easy/quick money, you will get the upperhand in the negotiating.
If it plays out that way, i have some ways for you to borrow money to make the purchase without having to sell your upstate place....keep us posted. goodluck