Lore question! Suppose, if the queen of Etrea had two daughters, then who would be Crown Princess? The elder one, or the more 'competent' one, even if they are younger/not well-liked? What about if there were two sons? Which one would be High General? Would being twins complicate matters?
In the case of two daughters it would be be the choice of the Queen. Naturally it would be the eldest, but it isn’t unusual for the Queen to make the decision to change who’s best for the throne. Alessia, for example, would choose the one more like herself and the Queen before her would have chosen the most well-liked daughter. So twins wouldn’t really complicate the line to the throne.
If it was two sons the Queen would be expected to choose a woman within her family, preferably immediate if she can, to take the throne. If there isn’t anyone within her family then she’d be expected to look at Matriarchs for each house.