Complex sentence: It was decided that in return for Louis IX supporting the Pope militarily, the Pope would allow Beatrice of Provence to marry Charles of Anjou, youngest brother to the French King, however Provence was to never go to France outright through Charles.
Split into simpler sentences: It was decided that in return for Louis IX supporting the Pope militarily, the Pope would allow Charles of Anjou, youngest brother to the French King, to marry Beatrice of Provence. But Provence was to never go to France outright through Charles.