I’m not really a fan of Kaneki or his parents being anything more than regular humans, but putting my bias aside, I don’t think it’s likely for Ken kaneki and Rize theory to be true given what we know.
For one, Kaneki and Rize are the same age, but their birthdays are a couple months apart with Rize being born October 8th and Kaneki being born December 20th. If these dates are actually true, which I don’t know why they wouldn’t be since they were given to us in the character info sections at the end of certain volumes, then that would automatically disprove the twins theory. But just for argument’s sake, let’s ignore the birthday thing.
If Ken’s mother was a human and his father a ghoul from the Washuu clan, Ken shouldn’t have been born a regular human. We still don’t know all the facts regarding human and ghoul hybrids, but in this case, Kaneki should have been born either a half-human or an one-eyed ghoul. Kaneki obviously wasn’t born a ghoul, and it’s unlikely that he was originally a half-human given that he was below average in terms of physical ability and all-around athleticism. I realize he wasn’t trained in the sunlit garden and he didn’t participate in sports or anything in school, but he still should have shown some signs in his life that he was half-human and had enhanced physical ability. Sure, we weren’t shown Kaneki in gym class in high school, but once he became a half-ghoul he still would’ve shown signs of unusual athleticism despite being relatively out of shape.
Also, Kanou said something along the lines of successful ghoulification surgeries not having much to do with blood type, beyond the normal requirement for most organ transplants and blood transfusions between two people to be of the same blood type. I could be wrong, but I’m pretty sure Kanou mentioned that of his thousands of attempts to create successful half-ghouls, he often tried to use people of the same blood type as Rize, and that only worked out 4 times that we know of: Kaneki, Furuta, Nashiro, and Kurona.
Now, it’s still possible that the theory is correct and Kaneki’s origin story, as well as Furuta’s taunting Kaneki about being just a random dude that happened to be in the wrong place at the wrong time, could be a big ol’ red herring of sorts. It could be viewed as odd that we don’t know anything about Mr. Kaneki and that he died when Ken was too young to remember anything, but it’s certainly not rare for a person’s parent(s) to die when they’re young. I suppose if Kaneki’s father was a ghoul/half-human from the Washuu clan/Sunlit Garden, maybe the reason he died was because he was murdered for breading outside the family or something, but I imagine Ken and his mother would’ve also been killed and/or Ken kidnapped to be trained in the Sunlit Garden.
I’m sure Ishida could spin this theory into reality in some fashion that I don’t totally dislike, but I’d really rather have Ken just be a random dude who rose to his current position and strength through sheer force of will, effort, and luck, and not partially because of familial ties.
TLDR: Kaneki would’ve been born a half-human or one-eyed ghoul if this was true, Ken and Rize’s birthdays don’t match up, blood type doesn’t matter a whole lot, and I hope this theory isn’t true.
P.S. I don’t think Rize and Ken’s mother look all that similar and the book thing is rather inconsequential in my opinion.