what can we do to get my fiance's mom to stop making false child abuse acusations to the local children and youth agency?

my fiance lost his son to children and youth 2 years ago because he made a mistake forgetting to put a gate up and his son fell down the stairs and broke his leg. he fought hard to get him back, and we are living together with with our 4 kids (some from previous marragies for us both) and his mother keeps calling children and youth for things that don't happen. a few months ago, she called and told them we starve these kids, and just today she called and told them my son had a black eye, and nothing is wrong with him. what can we do to make this stop, before we lose our kids? they mean everything to us, and we're not sure what to do. please help us.