Trying to find the title of a book. . .?

Question

I don't remember many details from the book. Here are the few i can recall - it's a suspense book. A boy and a girl [possibly brother and sister] are living with a man and a woman [i can't remember if the woman is their biological mother, but i know the man isn't their biological father]. The children are afraid that the man is trying to poison them in one part of the book, and they research different poisons and memorize the effects they have on the human body. There is another character in the book i remember - a young girl who had escaped from a children's correctional facility and befriended the boy. Toward the end of the book, the three children travel to an island [they ride their bikes to the edge of land, then a boat to the island] to escape the man, and he follows them there. I know this is EXTREMELY vague, but if it sounds familiar to anyone, please let me know. . . it's driving me insane! >.< Thanks for your time!

Answer 1

Sounds like a V.C. Andrews book.

Answer 2

Post your question on the Library of Congress' website at: loc.gov, if it was written it will be on that site.