I need help with aspects of my story

So…I'm back?
I left because I had a lot going on and I didn't want to stop to think about leaving because I probably wouldn't have gone. But it was a good decision and this won't be a permanent return.

Anyway, the reason I am back is that I have a plot idea and I can't figure out the part that is super dooper intricate and vital to the plot.
The characters aren't super fleshed out because my writing focus is on my current story (Compass Stars on Quotev) and also with making designs to turn into stickers to sell.

Here is the plot idea:
There is a princess who is well loved by the kingdom. She has a personal knight who is always with her who she likes but knows nothing will come of it. She was really close with her older brother, but he disappeared years ago and she believes him dead. When her mother passes, the princess becomes the queen. On her deathbed, the princess' mother tells her where to find her brother (who is not dead). Instead of telling her straight out, the princess has to decipher clues and goes on a quest to find her brother. Along the way, she and the knight fall in love.

So, it's clearly not very fleshed out but I have the main idea. There's no names or a title of the story.
But what I'm wanting help with is the clues. Since I don't have a very fleshed out idea, I'm not really sure what they should be. I think it would work best if there was one or two people who were willing to work with me on this for a little while. Full credit would be given to anyone who helps me.

Sounds like either a worldbuilding issue, or outlining to the rescue.

Worldbuilding because it's the world that has all the ingredients that you'll want to reach for. Establish that first as much as you can, and then add plot so that when you reach for specifics then those are right there.

Outlining because… while the growing affection between the knight and the princess are your focus, I think that if you follow the gist of the prince brother's journey first (not in prose, but in an outline format) then it'd be easier to write how your main characters essentially solve a mystery.

Because the fantasy sword-and-sorcery aesthetic might be obviously there, but the story structure sounds like a cross between a Fair Play Whodunnit (Agatha Christie style) and a "buddy" comedy.

It's less of worldbuilding and more of a "i just need someone more creative than me to help me make the clues"

Because it's not a main focus for me at the moment (like i said, I'm working on another story), I'm not going to spend a lot of time fleshing them out just yet. I actually think that will be easier once I have the clues to base the plot on and it will help me determine character relationships.

And the princess finding her brother is the main plot, with the romance a slow-burn subplot.

I actually think that will be easier once I have the clues to base the plot on and it will help me determine character relationships.

I can only wish you well in this story's endeavor, and in the one that you're currently writing and focusing on also. I hope you find the creative-process compatible co-writer that you're looking for! <3

Thank you!
It's kinda just been stuck in my head and I'm not creative in that way so I thought I'd come back here to ask.