I love these quiet backwaters of the Mudcat, far away from Punch The Horse or names of "thingies", where there are just a few people chatting about something that interests them. Somebody else will probably rediscover this thread in 5 years time and answer all miss_b's questions.
I only have the Topic/Caedmon/Rounder notes to go on and I've already noted all that says. From what I can find Raymond and Fred, as well as Aubrey and John Cantwell were all singers. But I don't know the exact relationships involved, nor the dates. However the Cantwell Bros. still have a family building business at Standlake, near Witney in Oxfordshire.
Peter Kennedy's book Folksongs of Britain and Ireland, gives three songs collected from the Cantwells, but no more information about them.
The songs are:-
Tam Broon, or The Cards from Raymond and John in 1956
The Husband-man and the Serving-man from Raymond and Frederick in 1956
The Nightingales Sing from Raymond and John in 1956 [??? but the same recording is credited to Ray and Fred in the Caedmon notes - with the memorable words about the whistling attributed to "seventy-three year-old Fred"]
Greg Stephens seems to have some more info about the family, and may be able to help out.