As a person that was not very nice to my sister growing up (but not nearly as bad as what some of these people are describing), I think as the parent you need to demand that they respect each other. Put your foot down and lay down real consequences for either child acting like a jerk. Because I didn’t realize what I was doing when I was a kid. But when I got older I apologized and wished that my parents told me to shut the fuck up or something every once in a while lol. They don’t have to like each other but they are siblings and obviously you are trying your best to raise good humans so I’d say they need to at least respect each other.