First, let us know if all of them, or how many of them, are speutered. If they have not been fixed, they all need to be separated immediately because they are of breeding age. If that’s the case, you would start by keeping them in separate houses with about 2″ between each one’s cage. You can do this with NIC cubes or xpens, so you don’t have to buy lots of cages. That’s essential until one month after each one is fixed. If you have two fixed first, for example, but not the others, you can wait one month and then begin working on bonding the first two. They do not follow rules like getting along with a bunny because they are from the same litter, or not impregnating their own parent bunny – they do it.
Here’s the good news in answer to your question: Once they have their individual spaces, it is MUCH easier to litter train them. Bunnies actually litter train best in a confined space. They can exercise in one room after leaving their cage, but it stays limited until they learn to go back to use their litter pan whenever they need to go. So, if you give each a litter pan, let them out only one at a time, and put all the poos back in the pan if they drop some outside the box, they will learn to use it. A bunny will pee (marking) or make a lot more poos if they smell another bunny because it’s territorial marking, so they are all doing that as well.