Would you think it was rape if the CEO of a modern business did this to one of their secretaries? Or if a parent did this to an adult child?
There are a lot of reasons for Thea to stick around and put up with the sexual abuse, even if she really doesn't want it, not the least of which is that her abuser is both her employer and a member of the ruling family in her country. Actually resting or leaving could end very, very, badly for her.
Similarly, the Princess clearly does a lot of good work when in public and the two have been together for a long time, with Thea being expected to take care of the princess. It shouldn't be surprising that she could admire her for her other qualities and might have developed a different form of love for her, despite her sexual abuse.
A parent? The princess is 5 years younger and doesn't even think she is more powerful than her guard even with her higher position, so how would that work?
So the princess was a rapist at age 10 already? She doesn't do it for power, she wants affection and provokes reactions out of the older one.
If your little kid sister touches you in an inappropriate way and tells you dirty things she heard somewhere, would you scream rape and call the cops?
I think you overestimate the position of the princess as well as the tone of the manga. :x
Literally everything you said was completely incorrect. Age is completely irrelevant, it's about power and authority, which the princess has and the guard doesn't. She doesn't need to do anything for that power, she just has it, and the guard can't go against her. If my little sister did any of that, there are a number of ways I could deal with it, all because I'm not obligated to stand there and take it. There's no overestimating the princess or misunderstanding the tone here, this is purely a power fantasy.