Hello and welcome to Binky Bunny!
Since your bunnies are 4 months old, you may have to separate them very soon, as they will be entering puberty. That can bring a lot of changes and the onset of hormones can bring aggression and that can lead to fighting and injuries. After their spay surgeries, then a month of recuperation and waiting for all the hormones to be flushed out, you can start rebonding them. Young bunnies can get along fine, but when they hit puberty, you can have trouble.
Are you positive that they are the same sex? Even vets have gotten that wrong. Many people have ended up with unwanted litters after being told that they had the same sex rabbits. So, that is another reason to separate them. But, even if they are the same sex and sisters, they should be separated until after their surgeries.
Here are 2 good articles on bonding:
http://rabbit.org/faq-bonding-multiple-rabbits/
https://binkybunny.com/BUNNYINFO/tabid/53/CategoryID/9/PID/940/Default.aspx