Thanks for playing! I was on a bit of a chess kick at the start of the jam, so when my first idea didn't pan out how I was wanting it I thought this could be a fun idea.
You make a good point about the UI, I'll consider some ways to better telegraph that you can use multiples of the same piece or remove them from their positions. It's intended that you can't move the two sisters starting positions (although you can take two of the big sister and have her on any tiles with enough stars). Showing how the mana worked in general was an issue as well, really just tutorials on how to play were kind of relegated to the end and then proper ones were cut for time unfortunately. There's an explanation in the controls section in the bottom right of the start screen but even there it could have been explained much better.
AI is another big one. It was tough trying to have it make moves that are at least understandable, but there's a lot of work that can still be done on it. On the bright side, I do have some ideas of stuff I want to do with the AI that might be fun.
The opponent placements also need some work. You're right that they're randomized, but they're separated into four difficulty sections that I thought would be of a similar difficulty, so you won't be fighting a queen on the first couple of fight for example. The main problem was that they were all basically eyeballed for difficulty level, so its definitely possible (and likely) that there's some inconsistent difficulty progression.
Not including check I don't really have a good excuse for... In the next version I'll probably add some kind of warning system for when you're leaving your king in check. The restart thing was an unfortunate bug, it was supposed to take you back to the beginning.
Thanks for all your feedback, I think there's a lot of improvements that I'll be able to make.