Mama Sis - Okaa-san to Watashi, Docchi ga Ii no by Boot Up

After the hero's mother died due to illness, he moved to Tokyo at the request of his father.
His home is the YAYOI Inn Pension (hotel), where Mireille lives and works, who looks after him.
Three years have passed since his father left Tokyo without a trace, Mireille became for him like a real mother, enveloping him with care and affection.
But inside they were alone, he lost his mother, and Mireille lost her husband a long time ago.
And to make up for each other's loneliness, they crossed the line.