It's misère tic-tac-toe, with a cute story!
It's kind of hard to intentionally lose (especially if I pay too much attention to trying to force lil bro to get three-in-a-row and forget to notice whether I'll get three-in-a-row, which happened multiple times to me). Lil bro isn't good at winning tic-tac-toe, but at least he isn't too good at losing, either; if he really wanted to lose (or you wanted to make this game impossible) there's a strategy he could have done which would have made it impossible for lil bro to win.
A couple minor issues: I wish when a game ended it stayed a bit longer and indicated somehow who won (like drawing a line through the three-in-a-row). Right now it just seems to show the final board for one frame before moving on, and a few times I won without realizing it and then was confused as to what happened. Also after the ending it just shows a blank cutscene, and clicking multiple times caused it to show some glitchy-looking text and then eventually the game crashed after clicking more times. (I'm playing on the web version.)