Poor Hubert Hedley seems to get his name mispelt all over the place!
I don't know how much time/money Sue Raftree/the RAF have to pursue this, but I'd say wills would be the way forward for the Robinsons at least - Hubert Hedley Robinson, OBE, I would say would be very likely to have had a will, which should list if there were any siblings of J.V. Robinson.
On the others:
The Times, Saturday, Mar 03, 1928; pg. 4; Issue 44832; col B and The Times, Wednesday, Mar 28, 1928; pg. 5; Issue 44853; col A record the divorce proceedings of George Owen Sharman and his wife Grace Evelyn Sharman, nee Rogers. They had a son and two daughters, but George was said to have committed adultery with a Elena Josephina Carolina Gabrielson ('Gabriel', the listed mother of Peter Siegfried?), and in return he accused Grace of adultery with Mr Lawrence H. Bradshaw, artist, and Mr Austin Osmond Square.
A decree nisi was granted, with the care of the two youngest children given to the father (they had been living in the same house as George and Elena for some time.)
Is this the same man? I can only find one George Owen Sharman birth on freebmd, 1871-1910, George Owen Sharman b. 1883, m. 1909 to Grace. If this is him, then I guess Peter had at least three half-siblings, and maybe some full siblings.