What happened to Marcus's wife in twilight?

Answer 1

Rather than explain this myself, I will leave this in the words of Stephenie Meyer herself =))

"Once upon a time, a fairly young vampire (he had only been a vampire for a decade and a half) named Aro changed his young sister Didyme, who had just reached adulthood, into a vampire in order to add her to his growing coven. Aro always wanted power, and because he himself had a potent mind-reading gift, he hoped his biological sister would also be gifted in a way that would help him rise in the vampire world. It turned out that Didyme did have a gift; she carried with her an aura of happiness that affected everyone who came near her. Though it wasn't exactly what he had hoped for, Aro pondered the best ways he could use this gift. Meanwhile, Aro's most trusted partner, Marcus, fell in love with Didyme. This was not unusual; given the way she made people feel, lots of people fell in love with Didyme. The difference was that this time, Didyme fell in love herself. The two of them were tremendously happy. So happy, in fact that, after a while, they no longer cared that much about Aro's plans for domination. After a few centuries, Didyme and Marcus discussed going their own way. Of course, Aro was well aware of their intentions. He was not happy about it, but he pretended to give his blessing. Then he waited for an opportunity to act, and when he knew he would never be found out, he murdered his sister. After all, Marcus's gift was much more useful to him than hers had been. This is not to say that Aro did not truly love his sister; it's just that a key part of his personality is the ability to destroy even what he loves in order to further his ambitions. Marcus never found out that Aro was responsible for Didyme's death. He became an empty man. Aro used Chelsea's gift to keep Marcus loyal to the Volturi, though not even Chelsea's gift could make Marcus show any enthusiasm for it."

This whole story is on her website under Breaking Dawn frequently asked questions. Hope I helped =))