Dear Mommy and Step-Mommy,115 years ago
Even though they are four I understand where you are coming from, try finding a common ground and expand on it,
Say they both like ponies, why not take them both to a farm to see the ponies and help them bond.
When they say they hate each other try to stop it. Get down to the two girls level and tell them in a firm but not angry voice that they should not say it because it is a bad thing to say,
Try nipping it in the bud before it gets out of hand
Best of luck,
Let me know how you get on