In reading about Mary, Queen of Scots I read that her two legitimate brothers (James and Robert) both died either on or about the same day (21 April 1541) but I can't find any information about how they died. Do we know how they died?
As for the cause of their deaths, to the best of my knowledge this isn’t recorded. The timing seems to be coincidence. Infant mortality was very high in the medieval era and kings were no exception. James V himself had three brothers all of whom died in infancy. I would be interested if someone has more details as it’s not something I’ve encountered in my reading about Scottish history.
Source: Lynch, M., Scotland: A New History, (London: Pimlico, 1992) pp.162-164