Knowing that Yotsuha and Mitsuha are actually related is news to me. I thought that Yotsuha was an orphan, and their "mother" who picked her wasn't her biological mom? I guess I missed that tidbit somewhere in the story.
If that is true, that does bring into question a few things:
1) Who is/was Yotsuha and Mitsuha's Dad?
2) Why was Yotsuha in an orphanage to begin with?
3) Did her "mother" track her down specifically?
Dunno why Yotsuha doesn't reveal about her abilities and the watch, because hiding anything from the loved one can and absolutely will cause issues in the relationship.
for some reason, i feel like Yotsuha is like a child of experiment or something, and even her step-brother strictly forbids Yotsuha from knowing the truth
It's quite possible that bro's explanation and handing over the watch (though we haven't seen the watch work again, not to mention he might have his own purposes) mean he's a red herring.
But I'm trope-weary enough to think it's just as plausible that the explanation is a red herring. Not all villains immediately go "MUAHAHAHA, you've caught me! Now let me reveal my entire evil plot to you so that you can eventually thwart it."