Is silence, on the part of the sister asking the question, an sin when she is capable of preventing her sister, who is studying Sharia, from screaming at and hitting their mother? And is it permissible to disavow this sister?

If a sister consciously and deliberately insults and beats her mother, this is a great act of disobedience (ʿUqūq) and a major sin. She must be admonished and prevented from doing so as much as possible. The father is not permitted to remain silent, unless she is excused due to a psychological or mental illness, in which case she must be treated.

The gravity of her sin should be explained to her, and she should be reminded of the importance of honoring one's mother (Birr al-Wālidain) and the consequences of disobedience. Her studying of Islamic sciences does not preclude this. If intervention would lead to a greater harm, then direct intervention (by hand) should be refrained from.

One should detest her actions, not her person, and wish for her guidance and righteousness. A believer should be merciful towards sinners, hoping for their guidance and repentance.