@missbluesunglasses As in the South they are a polygamic society, Graciela also hoped to marry the same man her older sister would have, because although she is a bit silly, Graciela really loves Berta. But since Berta was offered to the royal family as a wife that dream did not come true, so as a consolation, she can live in the city that her sister loves so much, with the man who was supposed to be her husband.
In the manga he does not mention it, but according to the web novel, one of the reasons why Berta wanted to live on La Marta was because her mother, the first wife of the Lord of Kasha, was from that city.