One thing I just noticed looking at the pictures, which never clicked with me before...at least, if Cunarders are going to share a platform with other lines, they're sharing it with lines of similar provenance.
Holland America - founded 1873 - home to such ships as NIEUW AMSTERDAM and ROTTERDAM.
P&O - founded 1837 - the Cunard of the Orient, with CANBERRA, ORIANA, etc.
Costa - founded 1947 (passenger service) - major player on the Europe-South America trade post-war, and had a fantastic roster of classic ships for decades.
So yes...QV and QE may be sisters to these ships...but of the Carnival Corp. brands to be sistered with, these are all ones with a significant maritime heritage.