Is it permissible for us to kill those who have seized our land and killed our brethren, while we have been expelled from among the tribes in Yemen for about 20 years and they are killing us?

Do not carry out the Hudud punishments or exact Qisas (retribution) yourself, for this is solely the prerogative of the ruler. Opening this door leads to chaos, killing, vengeance, and the loss of security. Do not lose your worldly life and your Hereafter for the sake of worldly possessions. Remember the Prophet's (peace and blessings of Allah be upon him) advice to stay away from fitan (trials/tribulations) and to be the servant of Allah who is killed, not the killer. Refer your matter to the responsible authorities and seek mediation from people of virtue and righteousness to obtain your right.