What is the ruling of Sharia on a girl disclosing to her sister the messages from her sister's fiancé, which included his disparagement of her sister, knowing that this might lead to her sister's divorce, and would she be considered unjust if she kept silent about it?

Do not tell your sister what her husband did, because you are a partner with him in the sin, and exposing the matter will not lead to any benefit; rather, its harm is greater. It is sufficient for you to apologize to your sister and to ask whoever you deem appropriate from your relatives to mediate, and that the matter involves a misunderstanding that does not need to be detailed.