Even if sei didn't accompany shigeru when he pees and create the butterfly effect, sei will most likely get into some shit later . His mother is the root of the problem. his mom crazyyyy 😔
P.S : his relative isn't that bad tho, even the aunt still think of sei and seiko when her own son is in a vegetative state, and his dad IS normal, he's just an average dad. If his dad was evil/terrible, his dad would've abandon him and seiko when she said she would have divorce him a long time ago in an early chapter.