Hi there, hope you're doing well!
Last year during December my mother decided to leave home without warning my brother and I. The police was alerted and a week later it was discovered that she moved to a different country with another man. I last saw my father left when I was 12, and I now live with my grandparents and aunts family.
Due to this I decided to retake year 12 and I'm currently studying 3 different subjects in year 12 because I would like to go to Cambridge. Do you think discussing this family issue as an extenuating circumstance count? I'm not using this as an excuse it's just a cause for my grades dropping.
As Avesta says, fill out an ECF. It seems likely that your situation would be considered a valid reason for retaking a year, which would presumably be frowned upon in normal circumstances. It would not, however, excuse poor grades (from the contextual data page: "It should be noted that we don’t use contextual data to systematically make conditional offers at lower grades, or to make allowances for a poor academic record.").