I don't think there's really anything wrong with what she's doing. It might be a bit mean, but so what ? Jing Xiu isn't aware that Ji Youyan got a chance to remake her life from the future. From her point of view the same woman who previously hurt and betrayed her just had a change of her from one day to the next and decided to act as if what had happened before wasn't.
And while Youyan has shown she's trying to be better, she also gives no promise or guarantees, shows no understanding of what Jing Xiu went through. She only knows she has fucked up, which she did. But this is not a bad test - because it expects of Youyan humility, understanding. To accept and get over a pointless bit of meanness just like Jing Xiu must if their relationship is to move forward.
If Youyan resents her for that, then she doesn't understand what she did wrong. On the other hand, if she truly feels sorry, she will accept it and commit herself.
Interestingly her sister isn't telling her not to date Youyan. She's telling her to test her, to keep some distance until Youyan's loyalty is proven. To not give her love easily because it would be taken for granted, which is exactly what the old Youyan, before her time travel, would have done and actually did.