Is a person considered undutiful to his mother if he demands his legitimate right to the inheritance after his mother and siblings divided the father's estate without his knowledge, and as a result, his mother supplicated against him for undutifulness?

Your demand for your legitimate right in your father’s inheritance is not considered disobedience to your mother, and no one has the right to prevent you from your right. It is permissible to take the matter to court for resolution in case of dispute. However, you must be dutiful and kind to your mother, and not mistreat or abandon her, no matter how much she mistreats or wrongs you; because the mother's right is great. Al-Bukhari mentioned in "Al-Adab Al-Mufrad" a chapter titled "Chapter: Being Dutiful to One's Parents Even If They Oppress." There is no blame on you for merely feeling alienated from her if you are dutiful to her.