Rabbits do not recognize family units, so yes, siblings will both attempt to breed and harm each other as a result of reproductive and territorial urges. While we have general estimations of when a male and a female become sexually mature, there’s no explicit sign and there’s such variation; therefore, it’s typically recommended to separate siblings earlier, rather than after they have a fight or have a chance to reproduce. Some rabbit vets require females to be at least 6 months of age before spaying, so waiting until both are fixed is not a good timeline. Hormonal behaviors can crop up as early as 12 weeks (potentially early probably), and even if viable reproduction isn’t possible (male doesn’t have viable sperm), the fights between rabbits are very dangerous. Even humping can be harmful, in terms of distressing one rabbit (the receiver).

I would recommend doing your best to help educate the person, if possible, or motivate them to learn more themselves. There’s a lot of preconceived notions people have about rabbit care. Sometimes, bombarding them with what they are “wrong” about makes it less likely they’ll explore resources on their own to educate themselves. It’s a difficult line, especially online when they may already be getting bombarded(?), but education is key with rabbit care, and having a closed mind usually doesn’t bode well for the pet(s).