Should I prevent my father from approving my sister's marriage to a young man with bad behavior, considering she is attached to him, or would I, by doing so, be sinning and depriving her of her destined provision?

The Muslim woman should not become attached to a person who is not pleasing in his character and religion, because religion and character are the criteria for accepting or rejecting a husband. This is based on the saying of the Messenger of Allah, peace and blessings be upon him: "If there comes to you one whose religion and character you approve, then marry him. If you do not do so, there will be tribulation on earth and widespread corruption."

She must also be keen to satisfy her father regarding marriage, as marriage is not valid without his consent. Compatibility is not a condition for the validity of marriage according to most scholars.

Therefore, the sister should be advised not to become attached to this person, and her father should be asked not to approve of him, especially if he is not pleasing.