Well, I think I heard that she died in a demon raid or something. Apparently Sparda "died" before the birth of Dante and Vergil, so if he was "dead" before they were born, she must have been helpless. Either that or he ran away or something, meaning Dante and Vergil were the only ones able to help her, and since they were young children (I think around five or six) when she died, maybe that's why Dante blames himself for not being able to help her and why he hates his dad. But your guess is as good as mine. I reckon Capcom have no idea what the hell they've done with Sparda. xD But they must have had some idea as to the whole plot in general because in DMC1 they mentioned Vergil's name when referring to Nelo Angelo, yet never made a reference to him being Dante's brother. Maybe they've plotted something and are teasing us. That's just sly.