What is the ruling on forcing one's sister to marry her cousin, and preventing her from marrying anyone else whatsoever if she refuses, and what is the effect of this on the marriages of her other sisters? And does the justification "the family inheritance must not leave the family" absolve one of sin?

It is obligatory to advise the girl to accept marriage to her paternal cousin if he is a man of religion and good character, for the Prophet, peace and blessings be upon him, said: "If someone whose religious commitment and character you approve comes to you seeking marriage, then marry him. If you do not do so, there will be tribulation on earth and widespread corruption." This offers benefits such as maintaining kinship ties and gaining the family's affection.

However, if he is not of that caliber, she has the right to refuse him and cannot be forced into it, and the condition imposed by her brother is void due to its contravention of Islamic law.

If another man proposes and she agrees to him, and her guardian refuses due to the invalid condition, she can refer her case to the judge to marry her. The same applies to her sister's daughters.