"oh no, we made a bad decision, let's demand that the other country concede the saintess back!"
Lady Philia cannot travel outside of Parnacota's capital city without her Ancient Magical Barrier dissipating. which, is a perfect excuse for those in charge of the Nation not to allow her to travel back to Gritonia. I am also unsure how that would work as Parnacota paid for Lady Philia (essentially she is a slave).
What are the chances ylthat the sister swap nations and dont grt a chance to meet
Never would happen as the nation of Parnacota respects Lady Philia and the second prince of Gritonia is in charge while his father the king is ill, and he is also now engaged to Lady Mia. Another reason would be that Lady Philia cannot travel outside of Parnacota's capital city without her Ancient Magical Barrier dissipating (perfect excuse for those in charge of the Nation not to allow her to leave the capitol).
dang, her parents are so unloving that she doesn't even realize that her sister loves her a lot even before she gets out of there
I am unsure if she feels that her sister doesn't love her as I didn't get that while reading through the chapters. What I did get is that Philia would sacrifice anything to make Mia happy and allow her to get whatever she wants as Philia has a sister complex (adores her little sister). However, I feel that once Lady Mia enacts her revenge on the second prince and her family (I will hypothesis here a little and say that what Mia is plotting is to reject the second prince in front of everyone when they are to be wed which will kill two birds with one stone (revenge on the prince and her parents (who wants to be favored by the royal family) for selling off her sister)) and she will run away to Parnacota to join her sister.