Complex sentence: Griffin and Cara's mother came in to be there for Cara and Tad's wedding, but really there was another reason for her being there: to convince Cara that this is not the best for her.
Split into simpler sentences: Griffin and Cara's mother came in to be there for Cara and Tad's wedding. But really there was another reason for her being there, to convince Cara that this is not the best for her.