The servants served Joseph by himself and the brothers by themselves. The Egyptians there ate with him by themselves

"The servants served Joseph by himself and the brothers by themselves and the Egyptians, who were eating with him, by themselves"

because the Egyptians could not eat bread with the Hebrews, for that is detestable to the Egyptians

This can be translated as a new sentence: "They did this because the Egyptians thought it was shameful to eat with the Hebrews"