It was a very common tradition of ancient Egypt's ruling families to keep the blood lines pure.
Why did king tut marry his sister?
Hippy is quite correct, it was not just the Egyptians, even all the way to Rome, some of the emperors married their close relatives (sisters included). Even in more modern times the rulers of Europe only married within a select group of ruling families. Often marrying first cousins. Although I do not know of a sister-brother marriages in the last 200 years of ruling families, I do not doubt there may have been.