Is it considered a sin for a married expatriate woman to remain far from her elderly and lonely parents? Is it permissible for her to leave her husband to live with them? What is the solution to avoid feeling guilty towards both parties?

Filial piety and kindness to parents, and obedience to the husband are both obligatory, and there is no contradiction between them. The questioner should reside with her husband and children, and show filial piety to her parents through gifts, correspondence, and communication, and by visiting them with her husband's permission and without any Sharia-prohibited acts. If they are in need of service, she should hire someone to perform it for them, if her financial circumstances allow.