I understand that rabbits quickly reproduce, but when a litter is born, is there a possibility that a male and a female from the same litter will reproduce?
All rabbits in a family can and will interbreed, sons, daughters, mothers, fathers, grandparents, etc. When two unaltered (not spayed or neutered) rabbits of complimentary gender (boy and girl) are put together for even a few seconds, expect baby bunnies 28 days later.
There are many variables but expect rabbits to reach sexual maturity in 3 to 8 months reference 1, reference 2. Unless you are intentionally breeding rabbits, consider 3 month old rabbits, old enough to make babies.
Babies can be weened at around 6 weeks if they are not separated or altered you can have a lot of rabbits in a short time If your are not able to reliably tell the gender of your rabbits they should be separated at 3 months old.
Speaking from experience, I can confirm that baby bunnies are born when two siblings of the SAME GENDER (believed to be) are left together. It is difficult to identify the gender of baby rabbits, and mistakes are often made. I volunteer with a couple of local organizations that take domestic rabbits, the majority of the babies that come in, come from just this scenario.