Maria was chosen to be the bride model because Yozora and Sena didn't want to become a model, and Yozora is against the idea of Rika become a bride for Kodaka To convince Pegasus, they went with the idea to use little girls childhood dream to marry their father or older brother, and Pegasus understand it though he confirm still asked if Kodaka is a lolicon
Neighbor Club then went to Kodaka house to train or let Maria feels what it is to become a wife. She cleaned Kodaka's room after Rika, Yukimura, and Yozora messed it for searching Kodaka's porn collection, went to supermarket to buy cooking ingredients (with Kobato too since she wanted to buy t-shirt (or if you choose certain choice she wanted to buy a pants, an erotic one XD), and preparing food with Kobato and Yozora. Maria, Rika, and Yozora then spent the night on Hasegawa house. Maria slept on Kodaka's bed while Kodaka himself slept on the floor. At night Kobato came because she wanted to sleep with Kodaka too.
The next day, on the photo session day, I'm not really sure but I think Yozora just realized Maria, a sister, is forbidden to get married and wear wedding dress. So then Neighbor Club asked Kobato to wear Maria's wedding dress since it already measured and Kobato is the only girl available with the same figure like Maria. Maria didn't mind it and Kobato at first agree, but on the chapel Kobato changed her mind, she against the idea to become the bride and locked herself on the club room. If you talked to Rika (and this is why I said you need other girls help to get a keyword/flag for good end of the girl you want), she told Kodaka the reason Kobato against the idea.
Kodaka then talked to Kobato and asked, like Rika told him, whether Kobato didn't want to do it for Maria's sake. Kobato said it's true because she felt sorry for Maria who can't become a true bride and wear wedding dress in the future, so she didn't want to take her role now. "You really are a great person, Kobato" "It... It's because I'm an onee-chan". Dawww ^^
The others heard their conversation and Maria herself thanked Kobato in her own way ("Heee, poop vampire is a good person"
), and that made Kobato felt really embarrassed. Oh, Sena was hold by Rika and Yukimura so she could shut up on this moment. In the end, the bride model is Maria. And on the photo session, they need the bride and groom model to do kiss scene and Kodaka was nervous, but the the cameraman said the kiss he meant was cheek only and that made Yozora, Sena, and Rika comment about Kodaka's being a lolicon. So they did the kiss scene and Maria was happy she could kiss Kodaka, though Kobato said that the kiss didn't count because it's only cheek
On the epilogue, Sena asked Rika to print a photo for her. I think the photo is a photoshopped one with Sena and Kobato on the wedding scene. Kobato ripped that photo and since Rika said she didn't have the back-up data for that photo, she ran outside the club room while crying. Rika then gave Kodaka another photoshopped picture of him as a groom and Maria and Kobato as his brides. Maria and Kobato fight for the picture but they accidentally ripped it too, and afterwards Rika said that she had a back-up for that one.