Complex sentence: This was a second marriage for Garcia as his first troubled marriage to Marguerite de l'Aigle had ended with her death just a month before Garcia's second marriage to Urraca that was arranged to strengthen Garcia's relationship with his overlord, Urraca's father.
Split into simpler sentences: This was a second marriage for Garcia as his first troubled marriage to Marguerite de l'Aigle had ended with her death just a month before Garcia's second marriage to Urraca. The marriage was to strengthen Garcia's relationship with his overlord, Urraca's father.