Is a person held accountable for feeling distressed by the persistence of the husband's mother in her demands from her only son, to the exclusion of his other siblings, despite the wife's admiration for her husband's devotion to his parents?

The mother's right is great, and honoring her is one of the best acts of devotion. What the husband does by spending on his mother is a righteous deed. However, the obligatory expenditure for the wife and children takes precedence over spending on parents. The mother does not have the right to demand from her son's money what would be detrimental to him or cause him harm or hardship in his livelihood. In this case, the husband can gently refuse her without insult, and honoring parents is obligatory in all circumstances. The distress you feel is pardoned as long as it is merely a feeling, but it is not permissible for you to prevent him from what is obligatory upon him towards his mother.