What if

I read a manga that had that happen in it and I was just thinking the same thing!

Nah, I think she is just a spoiled brat who thinks the world revolves around her. And who is starting to get really bothered by how Latte stole Armin from her.

Spoiler warnings, please. OP wasn’t necessarily looking for the answer.

I'm only guessing. I haven't read the novel or seen any raws.

I mean, it's pretty obvious by Ibelin's behaviour in the past few chapters that she is jealous of Latte.

I think that at first Ibelin was happy not because she got a female friend, but because she got another follower. As soon as Latte's attention stopped being all on her (when Latte was too busy running around to see the stuff happening or saving the background characters with ridiculous names) or helping her look better in front of the fishes (when Latte and Keith kept fighting for Ibelin's attention or when Latte was in danger and Ibelin got to pester Keith to go make sure she was fine), Latte was no longer someone Ibelin cared about. And once Armin officially stopped caring about Ibelin, she cut Latte off.

Of course, if Ibelin was a reincarnator, she could be one of those annoying ones who think they own ALL the love interests, and then all this jealousy would still make sense. But I don't think so. Mostly because she didn't seem to find strange that Latte, who was not anyone important in the original novel, was talking to her and wanting to be her friend. Even though the original Ibelin didn't have any female friends. Plus she knew exactly what to do in order to get attention when she wanted to (all the moments Latte thought it was weird that Ibelin was actively using her many skills to be able to stand out). Most reincarnated heroines, specially in light hearted stories like this one, don't know how to behave like a person of the world they got transported into and act according to our beliefs and social rules. That was not the case for Ibelin.

And of course, I could be wrong and Ibelin could be a smarter than most heroine, but judging by @Okarisu's reaction to my comment seems like I'm right.

Oh dear, I unintentionally spoiled it, hahaha. That being said, just because her motive is petty doesn’t mean she’s not smart and gonna make it interesting for us.