Hi there, welcome 🙂
You’ll want to separate them at 12 weeks to avoid pregnancy. The good news is that babies don’t really bond, so when you separate them it won’t be like breaking a true bond. Buns have been known to mate through cage bars, so you can house them next to each other, but you should have a bit of space between. You might be able to get your boy neutered a bit younger than the female, depending on when his testicles drop.
Separating them before hormones kick in and cause issues is the best way to keep them happy together, as well as getting them neutered/spayed as early as you can. I used to volunteer at a shelter and we have had quite a few sibling pairs come in. Sometimes after neutering and spaying they go right back together (even after being separated for several weeks), but sometimes they do need some work to bond them.
Since you are new to bunnies, feel free to check out the “Rabbit Info” section at the top of the page. There is lots of good info on housing, diet, bonding, etc, as well as some ideas for safe outdoor runs.
Also check out the info on RHDV (rabbit hemorrhagic disease virus). Not sure where you are in the world, but there is an RHDV outbreak in the USA currently, so it’s strongly recommended to keep rabbits indoors if you are in an outbreak area.
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.