What should I do if my father insists on having my young female cousin live with us in the same house, knowing that she is not mahram to me and I fear temptation for myself?

The basic principle is that there is no objection to your maternal uncle's daughter residing there, as long as it does not lead to an unlawful private seclusion (khalwa) or temptation (fitna). Speaking with a non-mahram woman is permissible when there is a need, provided certain controls are observed: lowering the gaze, absence of private seclusion, limiting the speech to what is necessary, and ensuring there is no suspicion or lust. The woman must not speak in a soft, alluring manner, should cover her body, and lower her gaze. The man, too, must lower his gaze and fear Allah. If you do not feel safe from temptation, then inform your father and try to convince him to reconsider. If he does not respond, then seek Allah's help and leave the house if possible. Otherwise, fear Allah and supplicate to Him to avert evil from you.