Complex sentence: She fled to Amesbury Priory, where she stayed under the protection of her aunt Mary de Burgh, who was a nun there, and where Elizabeth's posthumous daughter, Isabella de Verdun, named after the Queen, was born in February 1317.
Split into simpler sentences: She fled to Amesbury Priory, where she stayed under the protection of her aunt Mary, who was a nun there. There she gave birth to her daughter, Isabella de Verdun, named after the queen, in February 1317.