If a government became tyrannical and seriously violated people’s basic rights, would it still be sinful to rebel against that government?
The Catechism of the Catholic Church teaches in paragraph 1903:
Authority is exercised legitimately only when it seeks the common good of the group concerned and if it employs morally licit means to attain it. If rulers were to enact unjust laws or take measures contrary to the moral order, such arrangements would not be binding in conscience. In such a case, "authority breaks down completely and results in shameful abuse.
So, the short answer to your question is no. It is no always sinful to rebel against a government.
With that said, we must be very careful and prudent, always looking to the Church for guidance, in discerning when a government is enacting an unjust law and when it is not.