All court cases are extremely complicated. The aggravating and mitigating factors, the motive, and the precise role played by each person, in comparison to the others involved, all affect sentencing. Paul Mosley and Mairead Philpott were deemed to have played a lesser role in the crime than Mick Philpott, so Mick Philpott got life and Mairead Philpott (who was released three years ago, FYI) and Mosley got a lesser sentence.
Unless you were actually in the court room to hear the full details of the case and the aggravating and mitigating factors. You can't base a justice system on kneejerk reactions to headlines.