So the book is set in America in WWI around 1918 during the flu epidemic. I want to say Philadelphia or New York. The main character is a female teen or preteen. Prior to being sent to the country to escape the flu, there was a scene with the mother’s favorite tea cup boiled in milk to repair a break.
The girl is sent to live with an older man in the country. She gets sick and he has her shining vinegar as a curative. After she’s well, they collect peach pits for gas masks, knit scarves and socks for soldiers, sew gauze masks, etc for the war effort. The old man also fries fresh donuts as a treat for him and the girl.
I know they eventually become great friends, although she thought he was scary and grumpy at first meeting.