Writing Systems of the World 5, Exercise 3: tracing the family

From the Phoenician alphabet article: Hebrew, Syriac and Arabic derive from
Aramaic, which is a direct descendant of Phoenician. (The Abjad article adds
that Arabic developed from Nabataean, a form related to Aramaic; I did not
trace that step further.)

So the shared ancestor is Aramaic, and the script before it is Phoenician.
Phoenician itself developed from Proto-Sinaitic, which was derived from
Egyptian hieroglyphs, so the line runs hieroglyphs, Proto-Sinaitic,
Phoenician, Aramaic, then Hebrew and Arabic.