Maria passed away shortly after Sonia was born, in the hospital. She didn't recover from the birth well and had lost a lot of blood-- Maria was always delicate, and she just couldn't hold on. Sadly Sonia has no memories of Maria since she was literally an infant.

Cedric doesn't necessarily 'blame' Sonia for Maria's death, but a deep, dark part of his psyche does wish things had been different and that he still had his wife. He would never let Sonia know this though because he does love his daughter and does his best for her, but he'll always have that dark thought in the back of his head that if Sonia hadn't been born.... Well who knows.