Will a person be held accountable for their inner grumbling about their elderly mother-in-law who lives with them and whose demands are numerous?

What you feel of distress due to your mother-in-law's many requests will not be held against you. Serving her is a righteous deed, among the best of deeds and noble morals, which will weigh heavily in the scales on the Day of Judgment, and it is a perfection of kindness towards the husband.