honestly

This was hinted a lot through the story, that "mother knows her well". No matter how strange it is if You just look at it as decision agains her will, if You give it more depth and maybe a little unsaid story - this is quite reasonable to make her try marriage.
If You really are against marriage and are in that important position as her, given her strengh, she could easly refuse by any means. But her mother knew something and Arune couldn't say anything against for this idea. Later she was easly interested in this life so this prove my theory of some story behind it all (concerning mother and Arune).
And also - maybe Juleine love fighting as style of living, Arune not so much. She prefers to defend others for righteus things but given chance she's more like to not to do anything at all,just easly living life.

I don't know if I agree with it, because if she is in a position where she could've refuse getting married, she is also in a position where she could've refused even learning swordfighting and could even decided to be a "normal" princess instead (thing that it seems her mother always wanted her to do). But she did choose to learn it and even choose to become the twin sword's master, which means she actually have a interest (maybe a deep one) in it. So making her give up on that, disregarding all the years she spent training, all her work and effort, just because "mother knows her best".... I don't see it as a good (or even a true) argument.

I get that, there is a lot of "zero or one" thinking. But maybe, even if it suit her much earlier, she and her mother talk or went though some situation. And because of it mother became aware that this lifestyle could hurt her daughter more than Arune thought it could - there is mother story about longer lifespan and being alone. This is difficult to expect people (no matter how strong they are) to fulfill more than one role, but I think that her mother belive that Arune given chance to decide, she will go to the right decisions and won't regret a lot in future.