I think you should just write "Ottouto" and brother-husband above it as ruby text or furigana. There are many who are familiar Japanese family words like ne san, oni chan, They will enjoy the pun, also author uses this method with ruby text to put comments like love soldier san or imo no "best place".
Yeah, honestly I can't really most understand TLers' reluctance to include both description and rubytext at the same time. I don't feel it to be much of a hassle as someone who also did manga TL before. In case of this ottouto/hus-bro can just add bracket with actual meaning or if felt tacky, use TL note on the side of the page.
Heck, some of the most fire writing especially in this manga comes from the lore rubytexts themselves with how they describe horniness as actual valid powerlevels.
Miku's
Deception Memory (Fake Lover's Boasts) and Kaede's
Synchronic Memory (Remembrance Erosion) for example showed how far they had gone in their delusion about Sou to the point they made memories that didn't exist in the first place (at least in this timeline).
Which happened with Hana as well, come to think about it.