Complex sentence: The ulema have historically been led by the Al ash-Sheikh, the leading religious family descended from Muhammad ibn Abd al-Wahhab, who founded the Wahhabi form of Sunni Islam dominant in Saudi Arabia.
Split into simpler sentences: The ulema have historically been led by the Al ash-Sheikh, the leading religious family descended from Muhammad ibn Abd al-Wahhab. The latter founded the Wahhabi form of Sunni Islam which is dominant in Saudi Arabia.