Each individual with a HD parent has the same 50-50 odds of developing HD -- no matter what the status of their siblings.

In general, this data shows that juvenile and late onset HD cases are characterized by a shorter duration of illness (more severe); however, the figures for persons with older onset have not taken into account average life expectancy. The figures for persons with younger onset do not take into account the overmedication of juvenile HD patients. When these factors are taken into account the duration of HD is independent of onset and CAG count.
Duration v. Onset