What is the ruling on maintaining ties of kinship with one's sister and her husband who have abandoned prayer and hijab?

Prayer is the pillar of the religion, and abandoning it is a great sin, indeed, an act of disbelief according to some scholars. Similarly, not wearing the hijab is a major sin, and the husband's contentment with it is a crime. They should be advised kindly. Boycotting them is religiously permissible if it will affect them and prompt them to abandon the disobedience. However, if it will increase their aversion and transgression, then repeated advice without boycotting is better.