People in here are so far being quite unreasonable, I know that the main characters are the ones you naturally are the most accustomed to and thus empathise with but there is slightly more going on.
Think of it from the perspective of their moral standards.
Even if it is a different world Christina is part of a noble lineage and as such can be said to be under the stewardship of her father or brother until marriage and the transfer of that headship to the husband. Marriages are meant to be decided by the family and at a minimum have their permission. Men may be able to marry down but such a marriage is a lot more risky for women and thus a minimum level of status and a likelihood of advancement must be displayed.
His sister was taken into an unfamiliar territory by unknown means and then married without regard to family honour or tradition by someone of very low social status, an honourable person of low social status would have recognised the social differences and acted with deference and consideration, making sure to limit attachment. There were historically males of low social status who seduced upper class women and they were typically the male equivalents of gold diggers, having a very very poor reputation both in the upper classes and the lower (who they often treated a lot worse than regular nobles did).
As a society under constant existential threat strength is a key factor in deciding your place in the social hierarchy, with the social hierarchy in large part being designed to place limits on that power by creating obligations between the different social castes and thus directing violence outwards and limiting lawless rule by the strong, in medieval europe at least these were also intensely legalistic societies who viewed their social norms as as advanced and naturally correct as we do. As her fiance is a weakling he has no means of protecting her, not that that would have limited hostility much, the forced marriage and enslavement of women by foreign cultures has been a historical constant and was a source rather than a resolution of grievances. However his weakness should have made his level on the social hierarchy obvious and he should have understood that a lack of decency in dealing with her family would be a casus belli.
Lastly getting her pregnant means that her marriage opportunities are greatly limited, that she will birth a child of poor lineage and that a very extreme social taboo in all societies prior to the 20th century has been breached, in both medieval europe and indeed up until the mid 19th century the despoiling of a women (often regardless of social status though nobles typically made a grander affair of the thing) was a provocation which family and personal honour would demand recompense for, whether legal or in the form of the death of the offender, such an offence would very often be answered by a demand for a duel to the death, though in some places and especially when there was such a social difference straight up killing them was unlikely to raise eyebrows and with the circumstances even a jury of commoners would likely sympathise with the offended party and refuse to punish them. In East Asia the social difference would be a justification to kill them and their entire family lineage (e.g. everyone related to them by a certain degree of separation, e.g. three generations up, across and down or four or five etc).
Christina should understand all this as a person from their world and likewise understand that moving place does not break family ties or the obligations her brother has to retain his family's honour, she should also understand her brother's grief that she has greatly damaged her reputation and prospects and slighted her family, in sort the Asian concept of 'face' could be said to apply. The fact she doesn't consider this or attempt to close ground from common understanding is why she could be said to have changed, from the perspective of a clever person like the mage it likely doesn't endear her.
The brother has come from a great journey and been through much to ensure the safety of his little sister, he then finds that she has been seduced by a man of low social status with less strength and reliability than a cockroach, he did so in fragrant breech of family honour, personal regard and social consideration and as a weakling with no wealth or influence likely wouldn't even be able to take responsibly for it even if extended the opportunity. He has also consummated the marriage and thus greatly reduced the possibility for the situation to be salvaged. He then repeatedly talks to a social superior without having been spoken to and throws what he has done in his face while the man is still trying to process the situation. The fact he isn't dead shows extreme consideration by the brother (especially considering how much he would have had to have held back for that to be the case). His sister then fails to consider his feelings or the difficult position she has put him in and tells him he's of secondary interest to her, he makes clear just how easily he could have turned her husband into a red mist and huffs off in strop.
From the mages perspective she now has to mediate all of this while having regard to her own position and when Christina should already know what she did to upset her brother. The mage will be marrying the brother and as his wife to be she has several considerations, of course there is her personal attachment and sympathy to him but on top of that the power of marriage is a lot stronger in her society, by marrying him her interests will merge with his completely, from a legal and social perspective they will become the same person in a literal sense, as such any disagreements must be sorted out internally, with them presenting a unified front to the outside world. As his fiance she must fight his corner as if it's her own (the reason being that once she has children their position will be reliant on the success of her husband) and never publicly contradict or undermine him, preferably she should exceed her obligations and thus demonstrate (mostly towards other women as he's a catch, but also to the family in order to be accepted and towards society so that his own reputation is strengthened) and entrench her future position. As a result of this she has been left with a rather painful mess to clean up and her old friend Christiana doesn't even understand this and shows no consideration towards it.
As such she is left with the task of privately guiding her fiance, gathering information, dealing with outsiders, healing a family rift, finding a solution to family obligations which demand the death of someone important to the one you are trying to convince to return home, setting up and enacting a return, convincing said person to return, deciding whether it is even worth it if they do or whether it is better (in terms of both them and you socially) to act as though they died. As well as basically everything else as you have been left in a difficult social situation with your support being someone who is mostly good at killing things, who has personal skin in the situation and whom you can't contradict. If Christiana understood this and worked with her then the task would be a great deal easier but instead she talked about irrelevant nonsense, twisted the knife in the concerns of her brother and showed every sign of not understanding anyone's position, including her own.