I 'm fairly sure I have it figured out. I haven't read ahead, but the pieces fit together too well, so please don't read if you don't want to risk reading what's very likely a series of big spoilers. (Also, minor spoilage for Higurashi.)
Yotsuha does push herself too far and dies in one timeline… that she doesn't know about, because she didn't restart it. Notice how easily her older brother believes the premise of time travel? And how quickly he points out not to kill anyone for Uichi's sake, because Uichi would hate it?
He's speaking from experience. He blamed Uichi for her death, so he rewound (or whatever mechanism he used) back to farther than when she put the clover in the watch (chapter 41). He probably also caused their mother's death, but ain't no one gonna blame him for that. (He may have felt guilty because he's the one who went back in time to push their mother to go rescue Yotsuha.)
It might be a family ability, from their mother? Her reactions are often incomprehensible (e.g. how she treats Yotsuha, how she hears someone killed himself then says she guesses she doesn't need him anymore in chapter 40) — unless she's been trying to fix multiple time loops and her sanity broke under the strain.
I used to think this was straight-up "manic pixie dream girl tries to fix Higurashi"… but now I would bet the author first saw The Butterfly Effect and loved the idea of a family ability that breaks its users' sanity the more they use it (because accumulated trauma adds up fast).
I just hope her brother realized she's fighting him and is standing back to give her a chance to fix things, rather than that he's fighting her right up to the end like in Higurashi. But regardless, I get the feeling he loves her deeply… which is what twisted him far enough to do this, and why all his emotions and reactions to her are strangely blunted.
Mostly in line with my own theories for endgame as well, but it has some holes:
uichi dying or suffering it's what causes yotsuha to suffer and blame herself for his death, not caring for her own health and causing her possible death, which in turn would be the cause for the brother to become clover and go back in time, but without uichi suffering she simply goes back to him in December and his life hasn't gone to shit... So for revenge still being on the table there are two options: the brother is a serious brocon and jealousy is the motif of his awakening of those family powers, or uichi becomes some sort of an asshole to yotsuha, he forgot her, found himself a girlfriend, etc... Something that breaks her heart literally and bro takes revenge in her name, we need that first motivation for the bro to become clover or everything becomes circular, a loop