She’s not listed at Medlands as daughter of Gervais, count of Rethel & Elizabeth de Rethel / de Rosoy
http://fmg.ac/Projects/MedLands/ENGLISHNOBILITYMEDIEVAL3L-O.htm#Rob...
Robert [III] & his wife had two children:
1. ROBERT Marmion [IV] (-before Oct 1181). … m ---. The name of Robert’s wife has not been found. The Complete Peerage names “Elizabeth, daughter of Gervase, son of Hugh Count of Rethel” as the wife of Robert Marmion [IV] without citing any primary source on which it bases this statement[116]. As noted above, it is more likely that the daughter of Gervais de Rethel was the wife of Robert Marmion [III].
Robert [IV] & his wife had one child:
a) ROBERT Marmion [V] (-before [1217]). "Robert Marmion, fils de Robert" confirmed the donations to the abbey of Barberie made by his father by charter dated Oct 1181[117]. - see below.