This is an unfortunate situation for you. When you signed the bail bond, you were in effect taking out a loan to cover your brother's bail. The bonds company paid that to the court, and lost it when your brother vanished. They are looking for their money back, and you owe it to them - you signed the guarantee.
Unfortunately for you, even if your brother comes back now, the bail is already forfeit to the courts, and the company has no way to get their money back other than claiming it from you, as they acted on your behalf. You are at a loss of �50,000 because your brother missed his court date, and there's no way around that. If your only asset is your house, your house will be sold to recover the company's money, plus costs, and probably plus a hefty interest payment. Sorry, you're in a hole, and there's no way out of it.