From what transpired of the last episode (episode 3). The church seems to have been taking control of the power by basically controlling her brother. At the moment, the standing of the church is not super clear and we don't really know if they are involved in any of this.
From the characters point of view:
- When the king was alive and the country was strong, he got killed.
- Since her brother took throne and followed the church nothing has happened to him.
It wouldn't be wrong to imagine that by going back and taking the throne, she could either: do what the church says and stay alive or follow her father footsteps and probably get killed too.
I think it's a pretty smart move to wait few years and see the direction the country is taking before making any public claim to the throne. Also, being close to somebody who can raise a small army and control lands opens up a lot of options.
Judging by her past decisions, all this thinking has probably been done by the old man. I am convinced that, if she was given the opportunity, she would rush to the castle to ensure the well-being of her brother.