It's unpredictable. Male / female (of course neutered and spayed) works better than two females, even if the pairing is not impossible. Contrarily to what most people think being sibling matters really little to rabbits. They can get along initially and suddenly decide that they'd actually rather gut each other when the hormones kick in.
Also, be very careful : sexing young rabbits is not easy, there are a lot of mistakes and female rabbits can get pregnant really early (sometimes as early as 3 or 4 month old, which lead to a dead mother or / and dead kits).
In the end, I think it's easier to get an already bonded pair like blue eyes recommended or to get one baby, wait until their old enough to be desexed and then get another one from the opposite sex (preferably already neutered or spayed) to introduce to rabbit n°1.
Just in case you don't know : you will have to spay any female you get, intact female rabbits get phantom pregnancies, mastitis and uterine / ovarian cancer (80% of risks around 4 year old). The operation cost around 200$. Dont' take any female rabbit home if you don't intend to do this. That's not even talking about the behavioural problems (spraying red smelly urine, humping, territorial behavior including biting and scratching, leaving piles of poos outside of the litter box...)