In regards to Princess Ingrid Alexandra's 18th birthday comments: The princess praised her entire family (parents, siblings, grandparents, aunts and uncles, and cousins), as is usual on such occasions. Omitting one brother among the rest would have surely aroused speculations.
What she said about Marius in her official speech and her NRK interview was:
"Dear Marius and Magnus.
You are my safety net. I know I can always come to you when something is bothering me.
Marius,
Thank you for everything I have learned from you, and for being able to talk to me about everything. Thank you for always being there to protect me."
Speech given by Her Royal Highness Princess Ingrid Alexandra during the gala dinner at the Royal Palace in celebration of her 18th birthday, 17 June 2022.…
www.royalcourt.no
Interviewer: Does the Princess have any role models?
Ingrid: I have many role models, but I can name a few. […] And Marius, who I have perhaps always looked up to. Not necessarily because he is so cool, but more because of the loyalty he shows towards people. "
Thanks for the interview! She was nervous but did very well. And kudos for her to dodging tricky questions. Btw, I was also surprised to know that MM wants Ingrid to study medicine. I do agree that studying a variety of subjects can be beneficial, but I fail to see how studying medicine will...
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I think it is quite possible that Princess Ingrid Alexandra was simply expressing her honest thoughts. Many here interpret her recently leaked private Instagram post as a defense of her brother Marius, to the point of denial.
If that is true, the simplest explanation is that Marius, as a brother, treated his sister in a way that engendered such loyalty from her, despite their seven-year age gap and very different futures.
Being a safety net to her, being there to protect her, and being loyal to her (in the personal sphere, if not in an "avoid endangering her future throne" sense) would fit with that.