Year old comment but i feel like i know the answers

Why were they special that they got chosen to be reincarnated? Especially as brother and sister, to Ai?

Plague bearer seemed bored and lonely. After Sarina had wished to help heal and free a crow stuck in a net, plague bearer became interested in her. watching her, doing small kindnesses in her crow form. Putting a hat on her snow man and leaving interesting things for her to look at on her window. She watched Sarina wither away and die and saw Gorou devastated. She had one wish. to be born as the child of an idol. and with plague bearer seeing Gorou die and thinking the same.
Later on, although i cant pin the page or chapter, atleast in the localization it says "Given souls to lifeless children". Although it should have been evident from the start it would be a still birth and could be either a one panel retcon or a localization error. A god had ultimately taken interest in 2 people who did her a kindness and had the chance to grant them their dying wish. she enjoyed watching them in life and would have another chance to watch them again. Being a god must be boring, why let the show end.

If they got reincarnated then, why not now?

Because there was simply no need to do so. When things stagnated plague bearer gave ruby the push she needed. she was about to collapse and might have never stood back up. she appeared to give her a push, "Their killer isnt dead". and that is all that was needed to give ruby the chance to stand back up and keep on preforming for this bored god

What's special about Sarina/Ruby that only she can do, and that she absolutely has to be made happy and to survive?

At the start Gorou was the main character, however through out the series aqua slowly became their own. Gorou had 2 loves in his previous life that we know of. although we know from the scene with Kana that he did regularly go on dates through Kana pointing out he actually had reservations. that didnt compare to his two obsessions
Gorou idolized Al and loved Sarina (not romantically). He saw Al die and swore he would kill the man responsible, however slowly had the 2nd objective to let ruby live the life Al never could. to help help her achieve the goal she once had, to live as an idol like Al. In the mangas final moments Gorou dies before aqua drowns. aqua is an afraid 18 yearold child who wants to live, feel romance and be happy. there was a brief moment in time where aqua was shown, when he believed his father was dead. (During this time too the sparkle in his eyes disappeared. i dont know what the rules are for it). I strongly believe that canonically Gorous single minded obsession to kill his father and help ruby is the reason for the suicide ending.
however the suicide ended was clearly written on the walls since akanes breakup. the story simply couldnt end any other way. the author wrote themself into a corner and didnt know how to end it without aquas death. what way better to end it by having gorou achieve his goal and aqua drown in his namesake.

I do believe the super natural element of the story is simply a god being bored and wanting to watch something interesting play out. Gods like her are cursed with eternal life and need something interesting to engage with

promised yuri
lots of lillies in the outro

WHERES THE YURI???? =w=

Replace this with me :3