I think Marie choose to be on Rachel's side because, Arial's personality it's kind of domineering and looking down on people, it's not like Rachel's personality is any better. But if you compare Rachel to Arial she doesn't look down on Marie right away. Yeah, she doesn't give credit to Marie for her ideas but, she tries to get each and every information that's useful to her.
I thought if Marie was not a maid there. She would have been considered as a rival. But, she's working as a maid and she thinks if she becomes empress and have Marie by her side then, she can be acknowledged well and gain a lot of power.
That's what I thought
OR
The only thing Marie wants to do is move out of the palace as soon as possible. Her task was to choose one among two candidates picked as crown Princess. She had no third option..maybe she had but it would only delay her from leaving that place. So, she just picked one of the two that's kind of better.
I understand that Marie had agreed to being used by Rachel but... this just doest make any sense?? Like, I'm sure Marie had said she wanted to help the candidate that will support the Crown Prince. So why was Rachel chosen? Ok ok, Rachel is much better than Arial (?). But having Rachel be picked as Crown Princess isn't a good option either, she's using Marie's knowledge to gain power and once she is chosen for the thrown and Marie leaves, what then? She "suddenly" won't know what to do and might just call Marie back to use her again. So neither option would be "useful" for her.
TBH, I would've just stayed as the Crown Prince's maid than pick a side. Neither of them are worth the help and won't be useful to the Empire.
Thanks for reading my little rant.