What is the ruling on serving the husband's mother who is not ill, and is it obligatory for the wife to be with her daily since they live in the same building, and what are the wife's duties towards her?

You are not obligated to serve your husband's mother or any other of his relatives. You are also not obligated to stay with her, whether she is healthy or ill. Her care is the responsibility of her children. Your voluntary service to her is a part of maintaining good marital relations with your husband and is considered a noble deed.