well legally you have rights,
you can order an injunction to stop the marriage from taking place, this can be taylored to your situation and is issued by a magistrate.
in reguard to calling your partner over, you'll need to be 21, any applications made before this age will automatically be dismissed.
their are various projects and helplines you can call to get information. i do feel your very young. and i am also personally aganist marrying cousins not only because of the weaking of the gene pool, but also if one marriage breaks down all the family follows suit.
talk to your perents outline your beliefs, ISLAM dosen't allow perents to marry their children aganist their will.
inshallah you'll be ok