This is a SPOILER if আপনি haven't read the বই yet!!

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

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