Mahiro's Beach Fun (Part 2)

Mahiro's Beach Fun (Part 2) (まひろの海遊び（後編）, Mahiro No Umi Asobi (Kōhen)) is Chapter 58 of Onimai: I'm Now Your Sister!. It was originally published on Pixiv on November 27, 2021.

Summary
Nayuta accidently consumes the fast-acting feminization drug that Mihari brought along, causing her bust to grow tremendously.

Plot
Still at the beach, Mahiro notes that she is pooped and decides to take a break as well. Heading back to the base, she finds Nayuta sitting on the picnic blanket with her jacket on and her back turned. Mahiro asks Nayuta why she's hunched over like that, and Nayuta reluctantly decides to show Mahiro. Mahiro is confused, until Nayuta opens her jacket, revealing that her bust has grown tremendously after accidently consuming the medicine, much to Mahiro's shock. Mahiro notes that they are huge, and squishes them wondering if they are real or fake. Nayuta backs up and covers herself up in embarrassment, calling Mahiro a brute. Mahiro is shocked that they are actually real and is further freaked out when Nayuta explains what happened.

Mahiro decides to wake up Mihari to see what she has to say. After hearing what happened, Mihari is surprised that the medicine ended up in Nayuta's bag and apologizes to her, saying that it was her mistake. Mahiro adds that if it had happened to anyone else they'd be in real trouble. Mihari asks Nayuta if she's feeling sick, but Nayuta says that she feels fine except for her chest. She notes that they are heavy enough to stress her back and guesses that being busty has its drawbacks. Mihari feels a bit gloomy that she's now losing out to Nayuta as well.

Mihari says that the medicinewas a fast acting formulation, so it'll wear off quickly as well. She says that if things go well, it'll wear off in under an hour, much to the relief of both Mahiro and Nayuta. Mahiro says that they'll just have to keep it under wraps for a bit, with Nayuta saying that she'll hide it somehow.

Mihari notices that Kaede is starting to wake up and decides to wrap Nayuta in a towel and jacket to keep her bust from being seen, much to Nayuta's embarrassment.

Kaede wakes up, noticing that she fell asleep, but doesn't notice Nayuta's increased bust as the towel and jacket keep it under wraps. Kaede sees Mahiro, Mihari, and Nayuta looking at her and asks what they are doing. Mahiro and Mihari improvise and claims that they are doing the "awakening dance". Mahiro begins to tell Nayuta to do something while she still can, only to be interrupted by the return of Momiji, Asahi, and Miyo. Asahi notes that they are starving with Momiji suggesting that they eat something. All the while, Mahiro groans at them showing up at the worst possible time.

Once everyone has sat on the picnic blanket, Kaede tells them that they have yakisoba and rice balls and that everyone should just grab whatever they like. Asahi notes that yakisoba is great on the beach, but Momiji and Miyo sense that something's wrong. Miyo thinks that something's wrong with Nayuta, with Momiji thinking that she looks a little more filled out.

Mahiro sees the two staring at Nayuta and thinks that this isn't good. She whispers to Mihari that the two are already on to Nayuta, and Mihari whispers back that it is hard to hide something that big.

Nayuta starts to feel worse. She notes that her bust is becoming bigger and tighter, and that she can hardly breathe since its so tight. All of a sudden, her bust pops out, much to Mahiro's shock and Miyo's arousal. Mahiro realizes that the towel must've come undone, with Momiji noticing that Nayuta has suddenly gotten even bigger. Asahi doesn't know what's going on and makes casually asks Nayuta if she gained weight. Momiji and Miyo are shocked that she would ask something like that and scold her for it. Momiji tells Asahi that she can't just say that, with Miyo adding that that word is a no-no. Mahiro decides to use that as their line and claims that Nayuta is the type of person who gains and loses weight really easy. Nayuta backs this up by claiming that she puts on weight and loses it easily.

Nayuta decides to go and run some of it off, mainly as an excuse to get away until the drug wears off. As she runs away, Momiji notes that it seems like a lot of trouble. Kaede notes that it seemed like all that weight went to Nayuta's chest, but Mihari tries to tell Kaede that she's just seeing things. Mihari tells Mahiro that she'll handle things here and suggests that Mahiro go check on Nayuta, leading Mahiro to call this a handful.

Eventually, Nayuta runs what she considers to be far enough away from everyone else and opens her jacket to let her bust breathe a little. She notes that they're much harder to hide if they're big and decides to just keep them blow water level until they return to normal. As Nayuta goes under the water, she becomes shocked that her breasts float and notes that her buoyancy has increased tremendously. She thinks that she is sure that she can swim now and goes out further to test it out.

Mahiro sees Nayuta in the water and realizes that she is using it to hide. Much to her shock, Nayuta suddenly goes under, her theory having been disproved, and Mahiro runs in to rescue her. Mahiro pulls Nayuta from the water and tells her not to push herself, while Nayuta notes that she doens't understand why her theory didn't work. Momiji and Asahi show up asking Nayuta if she's okay, and Mahiro panics at the two finding them.

Momiji and Asahi are shocked at Nayuta's large bust, and Mahiro cringes having known this would happen. At that moment though, the medicine wears off and Nayuta's bust deflates, much to Momiji and Asahi's confusion. Mahiro struggles to find an explanation, so Nayuta claims that her bikini top is a trick swim top that makes someone look like they have a larger bust. Momiji and Asahi are relieved at this. Momiji says that it made her heart stop for a second, while Asahi says that she thought that something was up.

Mahiro is amazed that the two bought the lie, and that it was that easy to satisfy them. Mihari notes that it looks like Mahiro managed to keep them in the dark, though Mahiro doesn't feel satisfied. She notes that it makes her feel like her frends wouldn't even notice if her "comrade" grew back, with Mihari telling her not to press her luck.

A few days later, Mihari is in the bathroom looking at the mirror wearing nothing but a towel. She notes that she never expected the formula to have those kinds of effects if used by a cis girl, and decides to try it on herself "for science". Mihari takes the drug, and revels in the fact that she looks so busty now, staring at her reflection in the mirror. Mahiro catches her doing that and turns around in disbelief that her sister would do this. Mihari gets embarrassed and tells Mahiro not to stand there staring at her.

It is revealed that while still at the beach, Momiji looked at Kaede and wondered if her swim top was also a trick top, much to Kaede's confusion.

Characters

 * Mahiro Oyama
 * Mihari Oyama
 * Kaede Hozuki
 * Momiji Hozuki
 * Asahi Ōka
 * Miyo Murosaki
 * Nayuta Tenkawa

Trivia

 * The events of this chapter take place directly after the events of the previous chapter "Mahiro's Beach Fun (Part 1)".
 * With this chapter, Nayuta's theory that having larger breasts somehow makes someone a better swimmer is ultimately proven false. She first proposed it in "Mahiro and Swim Class (Part 1)".