Vicki! (1970)

A young nurse and a young man meet on a beach. A romance develops and pretty soon they fall in love. Immediately the girls mother is against the relationship and even goes so far as to meet with the boys father to arrange to have the boy sent away. He does this but the young couple still find a way to meet. They decide to elope and with the help of a friend they get married. The girls mother and boys father are in hot pursuit of the couple and finally catch up with them. It is then that we find out the young lovers are actually brother and sister.