Is it permissible in Islamic law to prevent one's sister from marrying a suitor due to a lack of academic, social, and cultural compatibility? Is compatibility a condition for the validity of marriage, or is it better to refrain from interfering? And how can one ascertain the sincerity of the suitor's intentions?

It is permissible to reject a suitor who is not satisfactory in character and religious commitment. However, if he is satisfactory, then he is suitable for your sister, even if she prefers him academically, because the considered suitability is that of religious commitment. Your cooperation with your sister in this marriage is permissible if the husband is satisfactory in character and religious commitment. If he is not, then you have the right to reject him and not to personally undertake the marriage contract, due to the saying of the Prophet, peace be upon him: "If someone whose religious commitment and character you approve comes to you, then marry him. If you do not do so, there will be tribulation on earth and widespread corruption." This is if there is no fear that her attachment to him is strong, such that she might be feared to commit what is forbidden or marry without a guardian. The man's condition can be ascertained by inquiring about him.