These villainess type storys really shouldn't be using the isekai tag, they are not isekai, they are in "other world" but it's not the normal isekai rehash, it's the normal villainess rehash.
Please don't get together with the prince. No matter how good looking and confident someone is, stay away from the crazy. 😭
There have been some spoilers in previous chapters' comments, and probably some foreshadowing already.
Not really a spoiler since I'm not sure, but I am making this guess based on other spoilers I've read. Anyway, I'm betting it's her brother from her past life.