See sources listed for Robert Clifford - none include a sister. Richardson's Magna Carta Ancestry says that Robert is the only son of Roger and Isabel (Vipont-13) Clifford.[1]
↑ Douglas Richardson, Magna Carta Ancestry: A Study in Colonial and Medieval Families, Royal Ancestry series, 2nd edition, 4 vols., ed. Kimball G. Everingham, (Salt Lake City, Utah: the author, 2011), Vol 1, p. 502, CLIFFORD 6.
Hi! With Burial place and birth/death dates, it seems likely that this John existed somewhere, just not as son of Roger and Isabel (Vipont) Clifford, but I couldn't find him.
John Clifford, 9th Baron de Clifford (Wikipedia)