"Well, if you really want to, then... okay..."

Momo and Ao are twin sisters who are living apart after their parents got divorced. After 10 years, Momo gets reunited with her younger twin sister, and she is fascinated by how big Ao's boobs have become, so Momo finds herself feeling her sister up while she sleeps... Ao is jolted awake by this, but she isn't particularly against it!

(Source: BookLive, translated)

Note: This has two editions, regular monochrome and official full color. Each edition also has two versions, one is a version with full censorship and the version marked R-18 has mosaic censorship. The covers are slightly different, but the story is the same across all four versions.