These women are not "grandmothers" in the conventional sense, but they fit the Gen'eki mold. They work constantly, date, and appear on magazine covers that would traditionally feature 20-year-olds. The fictional "wife’s grandmother" is an exaggerated extension of this real-life social phenomenon: the refusal of Japanese women to become invisible after 50.
Without more context, I can tell you that the title suggests a story focused on a mature, possibly older woman (the wife's mother) who remains very active and beautiful. The themes might revolve around family, relationships, and the exploration of mature intimacy. Tsuma no Sobo wa- Mada Mada Gen-eki Chou Bijuku...