All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, we ask Allaah to greatly reward you for taking into consideration the circumstances of your husband and for being kind with his mother; this is evidence of you having a good reasoning and wisdom.
Therefore, we advise you to continue to help your husband in being kind with his mother especially that you mentioned that she is very advanced in age. A person needs to be very diplomatic in dealing with the such a person due to her age, so you and your husband need to have a good understanding and be patient.
However, this does not mean that your mother in-law is permitted to interfere in your private life as she has no right to do so. Besides, it is not wrong to ask her kindly in a good and wise manner not to get involved.
If your husband fulfils his duty towards you but fails to fulfil something in which there is no obedience to Allaah as a way of being kind to his mother and in order to attain her pleasure and avoid making her angry, then this is permissible. However, he is not permitted to obey his mother in disobeying Allaah even if this makes her angry but he should clarify to her the mistake in a nice way.
Allaah Knows best.