Thanks for the chapter! this is oone of the mangas I'm most looking forward to.
Ao left it too late waiting until 23:55 , even if he hadn't heard whether his sister was ok before then , he should have started searching for a victim and re-wound the day anyway, since if he repeats the day he can always save his sister again but if he lets the next day arrive and his sister is dead then his sister is dead forever.
and even having let things get this late, he still shouldn't have started pointing scissors at them. he shoud have moved normally and picked up the scissors and acted like he was going to find something else to cut then suddenly stabbed one of them in the neck.
Or leave the room with the scissors saying you just need to go to the toilet and find someone outside.
But if he's in that situation where he can;'t leave and has to kill one of them, honestly it sucks but he probably needs to kill his mum because the detective as a fully grown man and police officer would probably be able to defend himself against a pair of scissors even if Ao attempted a surprise attack
it might seem not worth it to kill your mum but if that's the only way to save your sister then it is worthwhile since your sister is the continuation of your parent's bloodline it's the duty of parents to laydown their life for their children (so long as that doesn't prevent them from saving their other children)