Is there a sin upon the parents of a girl for forcing their daughter to marry her maternal cousin, and is it permissible for young men to pray for a specific girl to marry her, unseen?

Communicating with a foreign woman is forbidden by شرع (Islamic law), and you must refrain from it. There is no harm in supplicating for her to be your wife, but it is better to entrust the matter to Allah Almighty, so that He may choose what is best for you. Beware of excessive attachment to her, lest it lead to عشق (obsessive love). It is not permissible for the girl's family to compel her to marry someone she does not desire.