What should my sister do?

My sister has two kids. The father of her kids and her have been split up for 3 years now. He only gets the kids every other weekend. 2 weekends out of the month basically. He never gives her any type of money for the kids. She doesn't even coparent with him. She meets with her kids grandmother for pick up and drop off with the kids. She recently filed for child support because he's always going out , traveling, buying new clothes ,etc while my sister is struggling taking care of everything concerning them alone. He's now gaslighting her by saying he's gonna hire the best lawyer to get the kids taken away from her so he doesn't have to pay child support. She's scared of losing her kids. She's never had court for anything and she's a great mom. Any advice? Thank you!