"The June Sisters" appear to be a nationally well known variety act from the early 20's onwards. The sisters are named in "The Stage" newspaper (as well as other papers) as Grace and Ida more than once. I can't see any relevant births for the June surname so have to assume at least that part was a stage name.
Another scoop from Sc00p!
I don't think that Ida and Grace can be found in the 1921 census?

The June Sisters seem to have been in South Africa in 1921
Some snippets from the S.A. Pictorical (spelt like that) are on google books
Voume 12, 1921
"Smartly dressed, and presenting one or two novelties, the June Sisters, specialty dancers who also sing (as dancers apparently always must, though nobody knows why), provide a welcome note of variety. It is some weeks since a dancing-turn has been seen, and the June Sisters give a neat show, with the Mexican insurrection dance as the special draw."
I think there are couple of engagements for them in there.
There is an outward passenger list on ancestry/findmypast that could tie in!
Balmoral Castle, 31 Dec 1920, from Southampton to Cape Town
Mrs. E A Spiers Artiste 43
Miss I N Spiers Artiste 22
Miss G M Spiers Artiste 19