Frustrating Parents

I just got off the phone with my sister. As you all know, she moved to Florida a few years back and now I am responsible for making sure my family up here stays together. I am not an organizer. I am not the glue that will hold this family together. I'm lucky to hold my own little family together. She wanted to talk about my mother. My mom and dad got divorced like 3 years ago as well. Great time for sis to have moved. Anyway, so mom is not talking to her lawyer about my dad not paying her. They owned a company together and dad had to buy her out with monthly payments after the divorce. He's not living up to his part of the deal. My mom is struggling with her money because she isn't getting the payments. I need to call mom about it. Now how do I broach that?