Stephanie Meyer answered the question about Marcus's wife on the breaking dawn FAQ.
The story says that Marcus and Didyme (Aros sister) were in love and planning on leaving Aro. But Aro wasn't to fond of this idea and in order to rule the vampire would and keep Marcus with him he murdered his own sister. This is how I think the scene went
From Didyme's Pov
Aro's face had so many different emotions as he glided toward me. Anger, love, hope, hate, regret, and fear. I pulled my long blonde hair from my back to my right shoulder uncomfortably, as Aro continued you walk toward me silently. His eyes were looking at everything but me.
"Brother." I nodded ,greeting him. For a second his eyes flash pain but quickly turned back to being a mix of emotions.
"Sister." He greeted me. Something about the way he said the word sounded off, something was wrong, there was no doubt about it.
"What's wrong?" I asked taking a step toward him but he took a step back and focused on the ground. I had never seen him like this, he was usually tough and showed no emotion, even around me. His own sister.
"When I created you I thought you'd have a wonderful power." He spoke softly looking me dead in the eye. "But your power of being able to make people happy is, useless." His voice suddenly became sharp. "Marcus on the other hand," He paused. "Wonderful power. Very useful."
"Why are you telling me this?" My voice cracked. Marcus and I were planning on going off to marry tomorrow. Aro had told us he was okay with it but something was telling me otherwise. Aro loved power and Marcus had a power that could be very useful to him in the long run. But mine wasn't. I wasn't useful to Aro at all. He didn't care about me. He didn't need me.
"Did you honestly think I was going to let you take him away from me?" Aro asked walked closer to me but this time, it was me backing away. "Did you honestly think I was going to let you take all that power with you?" Now I knew what he was going to do. It was all so clear.
"He will never forgive you." I whispered. "How could you do this? Kill your best friends love, your own sister, just to get ahead."
"I have no choice, Didyme." His voice got louder, harsher. "Not true." I whispered. In one swift moment Aro had me pinned to the ground. He was stronger than me. Faster than me. There was no way I could save myself.
"A group of newborns came by," Aros face was so close, I tired to imagine his face from before. Before the power got him. Before he was a vampire. But I couldn't. "they were much too strong for you to fight off." He lifted me off the ground and slammed me into a wall. "That sounds convincing." He chuckled low.
"Marcus." I whispered. Thinking maybe he would hear me. Maybe he could stop Aro. Save me.
"He isn't saving you now." Aro whispered and it was the truth. It was too late.
"Make one correction to your story." I pleaded not looking him in the eyes. "You came and tried to save me but it was just too late." I flashed my eyes at him for a brief second, He looked amused. "And the last words you heard me say were," I shut my eyes tight. "I love you Marcus."
Then I felt unbearable pain.
Then I felt nothing. Nothing at all.