Makiko Nagaoka 
You Were Experienced, I Was Not: Our Dating Story 4th Date (Light Novel) [EPUB ebook] 

Winter is right around the corner, and Runa’s Friendship Project to help rebuild her relationship with her sister, Maria, isn’t making much progress. At the same time, Ryuto has to figure out an appropriate distance from Maria—she has a crush on him, and he still finds himself attracted to her despite being with Runa.
The main pair aren’t the only ones having relationship trouble either. Nicole’s joyous reunion with her past love quickly hits a snag, and Akari starts to have regrets of her own…
Meanwhile, Runa has something big planned for Christmas. What awaits our sweet couple as the year wraps up?