In the novel The Vendor of Sweets, Ambika and Grace are completely different in that Ambika was a traditional Indian wife whereas Grace is very much her own woman.
The late Ambika was Jagan's wife. It would seem that they had a fairly unhappy marriage, not least because Jagan was something of a domestic tyrant who insisted on being the dominant partner in the relationship according to the traditional Indian ways.
Ambika died from a brain tumor, and it is implied that her death was the result of Jagan's insistence that she use natural remedies. In any case, the fact that Ambika followed Jagan's instructions indicates that she was of a submissive nature in relation to her husband.
Jagan's daughter-in-law Grace is a very different person entirely. A modern, independent woman who knows her own mind, Grace exerts the kind of control over Jagan that would've been unthinkable coming from Ambika.
Grace exerts her control over Jagan by completely transforming his domestic environment as well as encouraging him to provide financial support for his son Mali's business ventures. Jagan's not used to being treated like this by a woman, and comes to resent Grace's presence in his life, especially after he discovers that she's not actually married to Mali after all.