The Idiom Attic - a collection of hundreds of English idioms, each one explained.
The meaning of "evil twin"
" Evil twin "
An imaginary double, humorously referred to in order to explain the uncharacteristic bad behaviour of a normally moral person. Usually used light-heartedly. The expression formed as an allusion to plots in films involving actual evil twins.
Jane's such a good girl and I took it as read that she would come to the wedding dressed appropriately, but she's turned up in full goth makeup. At first I thought it must have been her evil twin.