Must I fulfill the vow I made to myself not to enter my relatives' house until the girl and her mother leave, or to fast three consecutive months?

If you made a vow to fast for three consecutive months if you entered your cousin's son's house before the girl and her mother left, then this is a conditional vow that must be fulfilled if this condition is met. If you did not enter the house, or entered it after they had left, then nothing is binding upon you. However, if you entered it before they left, then you are obligated to fulfill the vow. And you know best what you intended by this vow.