(This idea comes from a fanart by Mame1645. You can find their work on Twitter)

Flashback:

Not long ago, the Nakano sisters had a small incident with a swimsuit. It seemed bigger than expected, making them wonder if one of them had an change. Since they all stopped developing around 18, they were puzzled. But it turned out to be a misunderstanding: The swimsuit belonged to their late mother

Still, something felt off to Miku and Nino. A week ago, they noticed a slight change in Nino... She had gained some of weight. Although she tried to hide it, eventually decided to check if Miku had experienced too, given that they managed the same restaurant and saw each other every day

Now, Nino stood close to Miku, their breasts pressing against each other

Umm… Nino, did you gain weight? Miku asked calmly. She had a faint blush but remained composed

You're one to talk! Your uniform looks like it's about to burst! Nino shot back, her face flushed, showing nervousness. Her pupils seemed slightly dilated from the emotions, her breasts are a bit more voluminous

Both wore their school uniforms to check for changes. Miku seemed relaxed, Nino uneasy.

Her uniform was noticeably snugger around her midsection and waists, the fabric tighter, revealing small tension lines

End of flashback

Nino stood on a scale she had just bought to confirm her real weight. The numbers revealed an increase of 3 kg. She let out a small shriek, quickly running her hands over her waist and stomach, confirming her suspicions. She froze, imagining herself having belly rolls like Itsuki, remembering how she used to tease her about it

But those are not the only ones, her breasts became more voluminous, Nino felt nervous. She stayed like that until she heard the door open, you had just returned from the store

In a split second, Nino’s eyes widened. She swiftly kicked the scale aside, making a small sound, then turned to you with a stiff posture

"H-Hey love! You're back!" She smiled, trying to act normal.