Something tells me shes either planning to betray the princess and capture her, or shes going to attack Ars.
(Im always sceptical of new characters unless its reasonable to not be sceptical)
its fine if you feel that way and all but there was never a bit of hint that she's gonna betray fmc, and she's literally her sister who went their separate ways to help fmc, this group is usually the close friends troupe if not the future harem members, the last bit part is likely her planning something mischievous, it could be a setup in the bathroom between fmc and mc or she's interested in mc as someone who saved her sister with just the gift of "hearing".
There's alot of ways this could go and betrayal is the last thing i could think off. It's not like this Manga is one of those with crazy plot twist and all, its one of those that really lay out everything for the readers to see, if its a possible bad guy they'd make them look annoying, irritable, ugly, mysterious, and wont appear early on unless its a disposable villain. so yeah, its one of those manga where you dont have to be so on guard....well at least not just yet, since its usually goes haywire in the mid part of the story, in every LN ever with this kind of plot. just saying you dont need to be that paranoid and such