My dad's girlfriend is interfering with my dad's economic decisions. When my parents separated the decided to split payments so my mom wouldn't be left with such a humongous bill that she can't afford.

He hasn't been helping my mom with the decided divisions in payments. He stopped paying most of the decided share. My mom can't pay it all by herself. I feel like I'm caught in the middle. And I have a younger brother he's only three and I'm fourteen. Can I have some advice please? What should I do?