This is a SPOILER if 你 haven't read the 图书 yet!!

No because although twins share DNA it is not all identical that why fingerprints are different. It is actually 更多 common to only have one twin with a mental illness.

Well, mental illnesses can happen if something happened to 你 as an infant. If 你 get shaken, for instance, your brain gets mixed up and 你 can have a mental illnes from that.