Is a woman who has performed the Hajj and repented obligated to pay expiatory feeding (kaffarat it'am) for the days she missed fasting in previous Ramadans, knowing that Hajj "returns the pilgrim as the day their mother gave birth to them"?

The noble Hadith, "Whoever performs Hajj for the sake of Allah and does not engage in sexual intimacy or commit sins will return as the day his mother gave birth to him," indicates that an accepted Hajj expiates sins, but it does not waive obligations. Therefore, your mother-in-law must fulfill the expiations that are incumbent upon her, for Hajj does not waive them.