These days, in AmE, "housewife" is disfavored as sexist language. "Homemaker" is a non-sexist term for someone who takes care of their own home, whether young or old, married or single, male or female. (In some couples, of course, the wife is the breadwinner, and the stay-at-home husband the caregiver. In such cases he might be jocularly referred to as a "househusband".)
But among the older generation, the traditional term "housewife" hangs on. So if a married woman has spent decades thinking of herself as a" housewife", far be it from me to deprive her of that title when she is left alone.