Complex sentence: Their daughter was Agnes Sigurdsdotter, who married the Swedish nobleman Jon Marteinsson (Stjerne) († ca.1400) and who was the mother of Sigurd Jonsson (Stjerne) († 1453) of Sudreim, Giske, and Bjarkøy.
Split into simpler sentences: Their daughter was Agnes Sigurdsdotter, who married the Swedish nobleman Jon Marteinsson (Stjerne) († ca.1400). She was the mother of Sigurd Jonsson (Stjerne) († 1453) of Sudreim, Giske, and Bjarkøy.