For those confused, while it is unclear who the mother of Watage is - What happened in this chapter is:
1) Tokio reacted horribly to being unstoned by Yamato, she cried for Kona, and basically rejected Yamato (from his view point) by mourning Kona not being there even though Yamato (HER YOUNG SON) who gave up his old life, lost his father, and risked everything to find her IS RIGHT IN FRONT of her. There is even a Panel where she BLAMES Yamato for saving her instead of being happy they reunited.
It's unclear how long Tokio whined and ignored her son to cry about Kona but someone pointed out that Tokio wasn't in the wedding panels (assuming Tokio isn't the wife).
2) It's unclear who the wife is, by looking at the wife's body it's probably NOT Tokio, but Yamato is likely using Tokio to turn his heirs into stone to continue the "lie" of the "stone princesses/goddesses" that the people around them worship to maintain his position as 'God King'.
3) Yamato probably was aware of Nekiji's feelings (this is just speculation on my part though) and something made him reject her indirectly with this wedding which probably hurt him (guilt)... and if it was politically motivated he probably felt like a total scumbag (He is feeding into the world of liars and becoming part of it).
4) Yamato is tired of being surrounded by liars and being a liar himself (he's a false 'God King' to maintain his kingdom), probably still emotionally damaged by the initial rejection of his mother, add the recent loss of Nekiji, and THEN seeing how loved Maru is by Kuroko instantly demanding that no one take the medicine if it could mean Maru would possibly die... Yamato decided in the spur of the moment "Fuck it, I at least want to see if I'm actually the REAL child instead of a fake."
It's easy to miss - Until he got triggered by seeing how "loved" Maru was, Yamato was demanding that Maru take the medicine... His flashback was just showing the reasons why he would make such a rash emotionally charged decision just for the sake of "truth".
I'm not sure where this "then they turn to stone and get kidnapped thing" will go, but with Yamato taking the medicine I guess this is pretty much a forced event if it's so important for us readers to not know who the real one is.
I actually forgot why it matters which one is real.