Is my not speaking to my parents considered disobedience and a sin, given that I am demanding they restore the rights of the orphaned girls after the unfair distribution of the inheritance?

It is not permissible to abandon one's parents. One must hasten to repent and resume ties with them, for their right is immense and is not forfeited by their wrongdoing or sin. If they have wronged others in inheritance, you must gently and courteously admonish them, as Imam Ahmad stated. There is no harm in seeking assistance from wise or righteous individuals to advise them to return the rights. Ibn Baz said that parents hold a great status, and one must treat them kindly and advise them gently, even if they strive to make one associate partners with Allah, citing His Almighty قول: {وَصَاحِبْهُمَا فِي الدُّنْيَا مَعْرُوفًا} (And accompany them in the world with beneficence).