I'm looking for a children's book series (6 books) each with standalone sci-fi stories, probably published in the 90s (latest early 00's).
The last (or second last) book was about a boy who always had a benign tumor but then started hearing voices, then discovered he had an unborn twin (named Kevin I think) absorbed in utero, effectively becoming the tumor. He enlisted the help of his classmate (Jamie?) to help infiltrate his parents' lab and found they made a synthetic body to transfer Kevin's consciousness into (scratching the surface of the skin on the hand to discover it was made of some synthetic material). He then gets knocked out and when he awakes he hears his parents calling him and Kevin, and his friend looking at him with horror, but his eyesight felt somewhat blurry. The book ends with him feeling some pain at the back of his hand and discovering that he got transferred into the synthetic body instead of Kevin.
My library classified it as a children's book (along with Animorphs and the like) but there were a few more complex words like benign tumor and epithelial.
I don't remember this one as well but the third (?) book was about a series of cloned girls with their names starting with A, B, C, D... etc. The protag (Eve?) found out about the cloned girls with shortened telomeres so they aged faster than normal people, and found articles online about the other girls dying from "old people diseases". She tried to get to the cure while developing arthritis.
I've been trying to remember this book series forever, so if anyone knows the title I'd be really grateful!