Is filing a lawsuit in court to reclaim rights from one's maternal uncle—who sold land belonging to the heirs under unjust conditions and colluded with the buyer to withhold their money—considered a severance of kinship ties, or is it permissible given the extreme need for money?

What your maternal uncle did—forgery and appropriation of your money—is forbidden (haram). It would have been more befitting for him to safeguard it and offer you good counsel. You have two options: The first is to attempt reconciliation by seeking assistance from people of good will. If he repents and restores the rights, then that is well and good. If he does not, the second option is to sue him in court. This is not considered a severing of kinship ties (qati'at rahim), for a son once brought a case against his father before the Prophet, peace be upon him, and this was not considered disobedience (uquq). If this is permissible with a father, then it is even more so with a maternal uncle.