Yup, after he jumps from that roof they find his suicide note.
Goodbye to each quint
Goodbye to his parents
Goodbye to his sister
What?
You thought that he was aright after the whole Rena thing and Nino/Itsuki shenanigans? Nope.
His wedding day is his dying hallucination, one where he gets a happy ending and starts his own family. But quints are his only female friends, that's why one of them is the bride. That's why we don't see ANY of the quints bar the "bride". He also only meets Maeda as he is the closest guy to being his friend in the whole school. His house is also how it was because he can't imagine it otherwise. Older Raiha - I bet that she looks almost like their mother did. Strange looking grandparents? He's dying, so of course there will be some strange things here and there.
Boom.
QUINTESSENTIAL QUINTUPLETS is one of the saddest stories around.