Welcome to the forum!
I will also ask, how old are the siblings? You saying that they get along nice means they're pretty young.
Why would you want to have babies in the future? Do you mean to fix and keep
taking care of them all? If not, i would highly suggest you not to breed your buns at all, because shelters are already overcrowded with bunnies and kitties and puppies, all looking for caring homes just because their parents' owners had an "oopsie" litter or bred just because they wanted to see the kits and pups grow. Neither of them is a good option, which still leads to a bad solution. There are more abandoned and homeless pets than there are good homes for them, so please think twice before adding on to that problem.
If you then do decide not to breed, i wouldn't think that the boy bun is needed in the sisters' bond at all- the sisters know each other well and if they get spayed at 5 months or depending on how soon the vet will do it, the sooner the better, they will have a higher chance of keeping that bond stable and strong, and then you wouldn't want a third wheel clogging the system.
Your sisters are adorable, but if you don't plan on breeding them for breed betterment as Apollo's slave said, please don't do it at all. They wouldn't care either way but they really don't need to have kits before spay.
If you want to see all sorts of critters grow up, and take care of them without actually having to worry "what happens if they're no longer as young as cute", then i suggest going to the local pet rescue and shelter to see if you can volunteer in helping the animals!