Suiyō Kyōdai

With her mother's recent remarriage, 26 year-old Tooko gained a new younger brother named Natsu, who is 13 years younger than her. At their parents' request for their own weekly time alone together, every Wednesday, Natsu comes to sleepover at Tooko's place.

While they still feel awkward due to suddenly becoming siblings, they begin to open up to each other as they spend their Wednesdays making and having home-cooked meals and mocktails together.

(Source: Kodansha, translated)