Sora no Neiro

This is the story of how a fake family becomes a real family. Kanon is a 29-year-old girl manga artist. She lives a lackluster life, half-hearted in both work and love. But one day, her sister and her husband die in an accident, and she ends up taking in her 5-year-old niece and becoming a parent...!? She's never even thought about having children. Not only is she unmarried, but her relationship with her boyfriend is half-hearted. And yet, she suddenly becomes a parent...!?

(Source: Flower Comics, translated)