didyme was aros biological sister who (when they were turned into vampires) became mates with marcus. she also had a power to make people fall in love with her (which is how i believe how the volturi got some of there gaurd), so when marcus and didyme decided to leave the volturi and live by themselves aro killed her so that marcus would not leave the volturi which is why marcus is so dreary and bored all the time (also marcus never new who killed didyme)
but no i am not really sure how aro killed her sorry but my best guess would be he probably waited until she was away from marcus then just turned her into ash lol
