For spoiler about cousin thing
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Aria's father is really judge frey sister however aria's father is not a royal of that kingdom but his sister is. Because their mother before married with the king(judge frey father), she had a boyfriend which is the marquis from another kingdom. So, king+queen got married - a son was born not long after that and later on they got a daughter - then 17 years ago, they found out that the son(prince that time) is not king bloodline but his mother ex boyfriend which make them kicked out from the kingdom but frey is staying there because she is actual royal bloodline.
Spoiler.
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No, they’re not related.
Basically, Aria’s paternal grandma married into the royal family but cheated with another high ranking person from Croa. So Frey is connected to the royal family (though not related to the prince) while Aria’s father isn't due to being from the external affair.
Here is link to fill spoilers of Aria’s lineage/family: https://forum.novelupdates.com/threads/the-villainess-reverses-the-hourglass.89566/page-9

If lady Frey's brother really is Aria's father then doesn’t that make them distant cousins?