Frying-Pan and Fire
It's odd that both Walter (William) and Robin contracted polio, though not of course fatally in my father William's case. It might be retorted that polio was a not uncommon hazard to both adults and children right through to the 1960's, so there isn't necessarily a familial link. But I think there might well be ...
Given their grandfather James' death from longstanding Tabes Dorsalis, and the epidemiological evidence that the disease can be transmitted congenitally to at least the following two generations (as was at least suspected even in those days), it is quite likely that both William and Robin had been treated precautionarily in early childhood by injections of arsenicals.
Such treatment is now realised to have had a collateral effect of increasing the child's subsequent susceptibility to polio, as per the following reference (needless to say I'm unable to access its contents, but the Abstract is pretty thought-provoking as regards the speed at which Robin succumbed):
Provocation of poliomyelitis by multiple injections
Transactions of the Royal Society of Tropical Medicine and Hygiene
Volume 79, Issue 3, 1985, Pages 355 – 358
a Dept. of Biology, University of Bradford, Bradford BD7 1DP UK
b Dept. of Bacteriology and Virology, University of Manchester, Manchester M13 9PT, UK
Injections of vaccines provoked paralytic poliomyelitis in children in the UK and elsewhere. The effect of multiple injections has not been recognized previously but could be important in the tropics where children receive many injections. A number of epidemics of poliomyelitis between 1914 and 1962 are related to children with congenital syphilis or yaws under treatment with arsenicals or penicillin.
Rates of 25% of children with paralysis occurred in epidemics while in non-epidemic periods the increase in susceptibility was about 25 fold. Other possible cases of provocation are discussed. Although in the tropics injections before paralysis may be causal, it will be difficult to prove that they are not coincident. The very high rate of paralysis following multiple injections is powerful evidence that injections in the tropics are often causal.