Complex sentence: Carl and Mitzee stop him from beating Riley to a pulp, however the Costellos then push Mitzee too far so she and Warren agree to destroy the Costellos on the day of the wedding.
Split into simpler sentences: Carl and Mitzee stop him from beating Riley to a pulp. However the Costellos push Mitzee too far so she and Warren agree they going to destroy the Costellos on the day of the wedding.