I think (and this is from my vague recollection of The Crown rather than any factual sources) that the Queen's mother had 2 nieces, daughters of her brother's wife with quite severe learning difficulties and then they had a few cousins who were also institutionalised too, however as these all came from the brother's wife's side of the family, it was assumed the gene for this was all in her side of the family and not passed on by the Queen Mother through her blood to the Queen etc.