Why did Sarah want Ishmael sent away from Isaac?

Les explains that Ishmael was making life miserable for five-year-old Isaac, and Sarah, ninety years old, simply could not take it. She did not ask that they merely move next door or go a half mile down the trail; she wanted them out of sight, out of mind. The Scripture in verse 10 says that because Ishmael taunted the son of promise Isaac, she told Abraham to cast out the bondwoman and her son, for the son of the bondwoman would not be heir with her son, even with Isaac. Les notes this was more than just the thought of a mother or a woman; it was the Word of God. Though Isaac and Ishmael later came together for the funeral of their parents, spiritually there was no union between them. Isaac continued in the spiritual line and later got a wife from Syria where the relatives were, while Ishmael married an Egyptian, showing a total divergence in the genealogies of these two young men.