Mrs. Martin’s Incomparable Adventure by Courtney Milan

Mrs. Bertrice Martin—a widow, some seventy-three years young—has kept her youthful-ish appearance with the most powerful of home remedies: daily doses of spite, regular baths in man-tears, and refusing to give so much as a single damn about her Terrible Nephew.
Then proper, correct Miss Violetta Beauchamps, a sprightly young thing of nine and sixty, crashes into her life. The Terrible Nephew is living in her rooming house, and Violetta wants him gone.
Mrs. Martin isn’t about to start giving damns, not even for someone as intriguing as Miss Violetta. But she hatches another plan—to make her nephew sorry, to make Miss Violetta smile, and to have the finest adventure of all time.
If she makes Terrible Men angry and wins the hand of a lovely lady in the process? Those are just added bonuses.