Is it permissible to ask my husband's family for help caring for their disabled son after his mother's death, knowing that I promised his mother I would care for him alone? And would this request be considered a breach of promise? And is what I feel—a nagging self-talk of distress concerning him—considered a sin?

Patience in serving the sick and the disabled is among the greatest righteous deeds through which a servant draws near to Allah, and it carries immense reward and great recompense in this world and the Hereafter. There is no harm in seeking the help of the patient's sisters to serve him, as they have the foremost right to care for him since they are his mahram (definition). His stay can be divided among them. Feeling distressed, annoyed, or wishing death upon the patient does not matter as long as it remains within the realm of inner thoughts and fleeting notions, and does not lead to an action or speech that harms him, provided there is regret and seeking forgiveness.