Complex sentence: A connection to the Taillebois family, if it was indeed one family, is mainly based upon a record in the Coucher Book of Furness Abbey, concord number CCVI, wherein Helewise, granddaughter and heir of William is party.
Split into simpler sentences: A connection to the Taillebois family, if it was indeed one family, seems to be justifiable as follows. The Coucher Book of Furness Abbey contains a concord number CCVI wherein Helewise, granddaughter and heir of William is party.