Is a woman relieved of any of the exhausting housework and service to her husband, if custom obliges her to do so?

All obligations are waived when one is unable to fulfill them, based on the Almighty's saying: (Allah does not burden a soul beyond its capacity), and His saying: (So fear Allah as much as you are able).

Therefore, whatever you are unable to do in serving your husband, or cannot do except with extreme hardship, is not obligatory upon you. However, what you can do with tolerable hardship is obligatory.

You should know that the foundation of the marital relationship is compassion and cooperation. A woman's good devotion to her husband is among the best deeds, for the Prophet, peace and blessings be upon him, said: "If a woman prays her five [daily prayers], guards her chastity, and obeys her husband, she will enter Paradise from whichever of its gates she wishes."