@mikl This is a typical use of https://tvtropes.org/pmwiki/pmwiki.php/Main/BedmateReveal and https://tvtropes.org/pmwiki/pmwiki.php/Main/WhatDidIDoLastNight as a humorous cliffhanger and even if there really is a other girl, then I doubt that it immediately turned story into yuri. With the same hope, you could expect Naruto and Sasuke to be a couple after that kiss, lol.
@KuroeNekumi Well, if we don’t get direct evidence that she was sleeping with someone, then we may well get the situation that someone undressed her and put her to bed. Anime and manga have used this cliché so often (hell, Listeners even parodied it) that these phrases can already be predicted with eyes closed. In any case, as long as we don't know any details, this will work perfectly for any assumptions.
UPD. Stop, why did you decide that oneesan left with male MC? It could also be part of her dream.