It is right for your father to come to your home. You are his son and your wife is his daughter-in-law. There is no harm in visiting both of you to enquire about your health. If your father had ever not uttered any word of talaq to your mother and they admit it, then it is right for your father to visit your mother to enquire about her health etc. No talaq takes place by long separation nor the relation of nikah comes to an end. Thus, your mother is his wife and it is right for both of them to meet in privacy.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband