Scarlet fever
The 16th Earl had 2 daughters; the youngest, Lady Gwendoline Catherine Talbot, died in 1840 aged only 22. This is close to the legend's alleged date range, but her death was not mysterious: she died of Scarlet fever in Rome. 3 of Gwendoline's children died of measles shortly after their mother, meaning the Earl had 4 deaths in his close family, not the single one described in the legend.