We had the same issue! Luckily, it has passed! Jaxon would literally attack Bri and I would have to intervene. I was so frustrated, but just like with any "siblings" you have to remain calm and still give them lots of loving. I about cheered the moment that Jaxon actually wanted to be with Bri. They can't be apart now, and even sleep together! Give them time
Good Luck!