Hellooo....someone please tell me....

I don't remember it was ever mentioned in the 아니메 또는 manga....
I know that Kuroba Kaito's father used to teach Shinichi's mother the arts of disguise, but I never heard that Shinichi & Kaito were cousins...(:

read chapter 570 to know all the truth

I don't know the answer, Originally thief Toichi and Yuusuku are frenemy. After Toichi's death Kaitou become KID... Later on, Shinichi and KID are frenemy too.

Yeah, I've read that too. But, I think they said like that because Shinichi and Kaito look like twins :D