Is stealing a letter from one's sister in order to prevent it from being sent to a young man considered haram, and what should be done about it?

If you spied on your sister, read her message, and took it, you have committed a forbidden act, because spying is unlawful. You must repent to Allah and seek forgiveness from your sister, unless you fear that doing so would spoil the relationship between you. However, if the message came into your possession without spying, and you learned what was in it and took it, then there is no blame upon you. This falls under the category of enjoining good and forbidding evil, as it is permissible to destroy the physical objects of evil. In all cases, you must advise your sister that such relationships with non-mahrams are forbidden and have dire consequences.