Praise be to Allah.
Advise your uncle to pray regularly and explain to him that the salah is the most important pillar of Islam after the Shahaadatayn. Forsaking prayer is kufr, and his fasting is not valid if he is not praying. His good deeds will not be accepted if he does not pray. Advise them both to be righteous and to fear Allah, and to uphold the ties of kinship and pay attention to the rights of relatives; whoever cuts the ties of kinship, Allah will cut him off, and whoever takes care of the ties of kinship, Allah will take care of him. Read to them both the aayaat of the Quran and the ahaadeeth of the Prophet which speak of that. Then if you uncle responds, then praise be to Allah, otherwise you have to shun him.