Sillius II was the son of King Guithelin and Queen Marcia, succeeded by his son Kinarius. Since his father, Guithelin, died when Sisillius was just seven years of age, his mother, Queen Marcia ruled Britain for about five years in his stead. Upon her death, in ca 358 BC, Sisillius came to the throne, ruling for the next six years.
- Monmouth, G. (Galfredus Monumetensis). History of the Kings of Britain. p. 101.
- Cooper, B. "History of the Early British Kings". Retrieved 2011-02-07..
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