Did the habit of "brotherly-marriage" and/or close-relatives marriage, play a part in the large number of Egyptian dynasties/short time of ruling for each dynasty?
You can't obviously compare this with Portugal, but in almost 900 years of history we had 4 dynasties. Egyptian on the other hand had almost ~10 per 1000years. Was this normal back then? Or was it possible that inbreeding diseases played a part/were a major cause?
Be careful with your definition of dynasties. Many of the illustrious ones lasted a very long time while many were only one or two pharaohs long. There were also many overlapping dynasties during the intermediate periods.