What is the ruling regarding my sister and mother severely beating my sister's son, depriving him of play, and subjecting him to psychological and physical torture, and are my siblings and I sinful if we remain silent about this?

Your sister is abusing her son by hitting him, and your mother is helping her in that. Hitting a child at this age is a great corruption. You must prevent them from this evil (munkar) and advise them by clarifying the impermissibility of their action and the harm that results from it. Inform them of the words of scholars and specialists in upbringing.

If that does not help, inform the child's father or anyone who can prevent this harm, even if it leads to raising the issue with the court. It is not permissible for you to remain silent about this evil (munkar). If you do not change it, despite your ability to do so, then you are sinful.