What is the Sharia ruling on dealing with one's parents and sister when they are shunning him, and the father is refusing to give the son his rightful inheritance while having given it to his brothers?

Maintaining kinship ties is one of the greatest obligations, and severing them is a major sin. The true maintainer of kinship is one who connects with those who have cut him off, and rewards those who have wronged him, not merely one who connects with those who connect with him. It is not permissible to shun your sister. If you are harmed by her actions, you may reduce your interactions with her, but not cut her off completely. For one whose relatives cut him off while he treats them well and they treat him badly, it is as if he feeds them hot ashes, and he will have a helper from Allah against them. You must mend the relationship between you and your sister to preserve kinship ties and to please your parents. If your parents' estrangement is due to your shortcomings towards your sister, this will disappear with reconciliation. However, if it is without reason, it is not permissible, because justice between children is obligatory. Mistreating parents does not justify the child reciprocating in kind, for honoring them is obligatory in all circumstances.