listen, isabella was jealous of rosaline/escalus. she loved rosaline, not only as a friend but as someone she wished to kiss, and when escalus and rosaline were so close - joking about marriage and the future and love -
isabella was not only jealous that escalus would get to be the prince, would get to rule, would always be their father’s favorite - escalus had rosaline.
isabella likely pushed her father along. accidentally mentioning those stable visits, suggesting escalus get education elsewhere.
but rosaline’s dad dies. and then her mom dies. and suddenly, rosaline has been taken in as a servant - and isabella knows that as princess, she’s not supposed to associate with them.
when rosaline reaches out, isabella has to remind her that there are rules.
and when rosaline never talks to her again, isabella has to remind herself that this for the best.