What should be done with a much-troubled uncle who accuses his siblings of theft and refuses to apologize? Is it permissible to shun him? If shunning prolongs, does it become a severance of ties, and does its sin fall upon everyone? What is the ruling on obeying a father who forbids communication with him?

The actions of the uncle mentioned are not permissible in Sharia, and he must repent from them and cease accusing his brothers and encouraging his children to misbehave.

As for his brothers and their children, they must advise him and not totally abandon him unless associating with him results in harm to their religion, lives, or wealth. If the harm is specific to some of them and not others, then the abandonment is specific to those who are harmed, and the others should not abandon him.

The least that severs abandonment between Muslims is exchanging greetings (giving Salam).

As for relatives, merely ceasing abandonment is not sufficient; rather, maintaining ties with them is obligatory. Therefore, if you are not harmed by maintaining ties with your uncle, you must do so, and there is no obedience to your father if he prevents you from it.

You can conceal your ties with them from your father to avoid his anger.

Maintaining ties is achieved by everything that is customarily considered as maintaining ties.