Is it permissible to pay off my brother's debt from Zakat money, knowing that he took these funds from some people without legitimate right, and a lawsuit has been filed against him?

Your brother is a disobedient sinner and usurper of those funds. It is obligatory for him to repent and return them to their rightful owners. He should not be given from Zakat unless he repents and is unable to repay the debt. For his repentance to be considered sincere, a strong presumption is sufficient, even if a long period has not passed.