Based on Medlands and the use of the name elsewhere, Aelfthryth is her father, not her mother, as it's a male name. She was daughter of ÆLFTHRYTH & his wife ---
http://fmg.ac/Projects/MedLands/ENGLAND,%20AngloSaxon%20nobility.ht...
MORCAR (-murdered Oxford summer 1015, bur Burton). King Æthelred II granted land in Derbyshire to "Morcar minister" under a charter dated 1009[218]. With his brother, he was one of the leading thegns of the northern Danelaw. Simeon of Durham records that "Sigeferth and Morkar the sons of Earngrim" were killed in 1015 on the orders of "duke Edric Streona" and that the king took possession of their estates[219]. The Historia Fundatoris of Burton Monastery records its foundation by “Consul ac comes Merciorum dominus Wulfricus Spott regali propinquus prosapiæ” in 1004, adding that "…comite Morkero cæterisque cognatis eius" were buried there[220]. m (before [1002/04]) EALDGYTH, daughter of ÆLFTHRYTH & his wife ---