Why did people in ancient Egypt marry their siblings?

Yes especially when inheritance was involved. Although it was a male that iherited it followed the female line. The eldest son of the eldest daughter. In other Words it was nephews that inherited rather than sons. So if you were a man and you wanted a future son to inherit your property you had to marry you sister and have a son by her. So you son was not just your son but also nephew It was quite common for the Pharaoh to marry his sister and I am my own grandpa!