Why is the Sinai covenant compared to Ishmael?
The Sinai covenant is compared to Ishmael because it was given through Moses to Israel, God's earthly people who have earthly promises. That covenant from Mount Sinai gendereth to bondage, corresponding to Hagar, Ishmael's mother, so it pictures an earthly, bondage-producing covenant rather than a spiritual one.
Les explains that these things are an allegory of two covenants: the one from Mount Sinai gendereth to bondage, and that is Hagar. After Israel came out of Egypt and encamped around the mountain, God called Moses up and gave him the Law, the Ten Commandments, and all the rest. Moses came down and gave the Law to Israel, so God used a man to mediate it. Les points out this is a comparison with Ishmael because Israel is God's earthly people with earthly promises, and everything concerning them is earthly. Thus the Sinai covenant pictures an earthly, bondage-producing line linked to Hagar and Ishmael.
Which things are an allegory: for these are the two covenants; the one from the mount Sinai, which gendereth to bondage, which is Agar.
For this Agar is mount Sinai in Arabia, and answereth to Jerusalem which now is, and is in bondage with her children.