Be happy too explain why I think Itsuki is the bride.
Chapter 1- First girl Fuu calls his Partner.
Chapter 2- Rahia approval - Fuus sister is very important too him. Any bride is going to need her approval so far that's Itsuki or Yotsuba.
Chapter 6- defends him against Nino. Even though she dislikes him, she uses logic instead her dislike to get rid of him. ( this is less a point than the first but it's still worth nothing )
Chapter 7 - More Rahia approval. Especially the real family line.
Chapter 12 - Nino mentions Fuu left Itsuki in tears
Chapter 16 - The Argument is a lovers quarrel, it's far more intimate than the spats him and Nino have.
Chapter 18 - Chooses Itsuki over miku, Ichika and Yotsuba.
Chapter 19 - Itsuki Stands up to nino letting guy stay the night before exam day, she also offers him food for the trouble she's caused.
Chapter 20 - Refuses to let him resign.
Chapter 23 - comes to collect him for the school trip.
Chapter 30 - Thinks he's Superior to their father. Fuu takes her to be alone on the stair lift.
Chapter 31 - first to sneak into his room when he's sick ( potentially )
Chapter 32 - first to hold his hand. The rest hold his hand together later.
Chapter 33 - Dreams of Rena when Itsuki is in the hospital room.
Chapter 33-34 Tells Itsuki about Rena, probably his deepest most personal experience.
Chapter 39 - Itsuki worries about him, then stands up to Nino. Fuu invites her
Chapter 40 - More Rahia bonding. Of all the options Itsuki chooses to go to a house of somebody she supposedly dislikes.
Chapter 41 - more Rena discussion, and the arrival of Rena again
Finally chapter - 42 Rena retuns, while it's revealed in revealed in 79 that it's Itsuki. This cemented Itsuki as the bride.
Chapter - 61 both yell of the cliff together
Chapter - 63 the discussion in the hot spring. Another meaningful talk
There's more but that really should be enough.