Synopsis
On the day of her hot spring trip with her boyfriend Yuuya, he suddenly cancels due to work. Her step-brother Hibiki immediately rebooks the reservation, and the two end up going on the hot spring trip together instead.
“This is just a family trip”—the protagonist tells herself this, but deep down, her heart wavers with feelings for Hibiki that she’s come to acknowledge.
Although she worries about her future with Yuuya, her current status as “someone with a boyfriend” remains unchanged. She resolves tonight: “I won’t be swayed by emotion…!”
Meanwhile, Hibiki also tries his best to behave as a proper “younger brother” so the protagonist can relax.
However, when he sees her sleeping defenseless in a partially undone yukata, his reason wavers. Before he knows it, his hands are reaching toward his own body—
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105 pages main + cover + afterword 1 page + announcement 1 page
Total: 108 pages
※Censored with white lines
※The protagonist has a boyfriend, so this contains NTR elements.
※This is the sequel to “I Don’t Know This Brother” and “I Don’t Know This Brother 2.”
Reading the series in order is recommended for full enjoyment.
Author X (formerly Twitter): https://x.com/hanyonmll
Get “I Don’t Know This Brother 3 – ” on DLsite
This Week’s Top Rankings:
Editorial Review
This is the third entry in a step-sibling romance series that has found reliable commercial success by anchoring its appeal in forbidden-relationship tension and emotional conflict rather than pure mechanical eroticism. The hot spring setting is genre-standard for this subgenre, but the synopsis positions this installment around a more interesting psychological negotiation: both characters actively *trying* to maintain boundaries while circumstances and desire conspire against them.
What distinguishes this volume is the deliberate staging of internal resistance. The protagonist’s acknowledgment that she has romantic feelings for Hibiki while remaining technically committed to her absent boyfriend Yuuya creates legitimate narrative stakes—this isn’t casual infidelity but a character actively wrestling with competing loyalties. The tag combination of age gap, step-brother dynamics, and NTR (the last signaling her boyfriend’s implicit displacement) targets readers who derive tension from transgression layered with emotional complexity rather than straightforward taboo violation. Hibiki’s parallel struggle—trying to be “proper” before succumbing—mirrors her conflict and suggests the work invests in both perspectives rather than treating him as a purely aggressive force.
The 105-page count positions this as substantial for the category, and the shift toward mutual vulnerability (nipple play, masturbation framed as loss of control rather than dominance) suggests a more intimate, almost tender approach to intimacy within the forbidden framework. The yukata imagery and hot spring isolation provide effective narrative justification for prolonged, unguarded moments.
This will resonate most strongly with readers who’ve been following the series and those who prioritize emotional ambiguity and mutual desire over power imbalance as their primary appeal. The work’s persistent focus on the protagonist’s internal conflict—rather than her boyfriend as an active antagonist—keeps the emotional complexity genuine.
A solid continuation that understands its audience’s appetite for guilt-adjacent romance with substance.
Related Tags:
NTR | Masturbation | nipple play | hot spring | Age Gap
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