Wikipedia
Świętosława was a Polish princess, daughter of Mieszko I of Poland and Dobrawa of Bohemia, and sister of Boleslaw I of Poland. She was married first to Eric the Victorious of Sweden and then Sweyn Forkbeard of Denmark, according to German chroniclers, and gave them the sons Olof, and Canute and Harald, respectively. The name is only known through an inscription that gives the name of Canute's sister and the assumption that this sister was named for her mother.
The Icelandic sagas give her rôle to Sigrid the Haughty. This account is not considered reliable by scholars such as Birgitta Fritz, who assign greater reliability to the contemporary chroniclers. Snorre Sturlasson also mentions a Slavic princess he calls Gunhild of Wenden.
Researcher Rafał T. Prinke in "The Identity of Mieszko I's Daughter and Her Scandinavian Relationships"(Roczniki Historyczne LXX (2004),[summary in German], Poznań – Warszawa 2004, ISBN 83-7063-429-X, pp. 81–110) establishes that Sigrid the Haughty (Sigríð Storråda) was the daughter of Skoglar Tosti, while the name Świętosława belonged to the mother of Mieszko I of Poland and his granddaughter by daughter Gunhilda (her mother being Mieszko I's second wife, Oda) and her husband, Sweyn Forkbeard. This would make Świętosława the sister of Canute the Great.