I’m raising 3 littermates for the first time, how can I avoid littermate syndrome?

Hi! I recently had a big litter of 7 puppies (by accident, the mother is sterilized now 😅) 2 months raising them and me and my dad gave them away 4 of them, we were going to keep only 2 (two girls) but my dad wants to keep the third ( a male), I have been reading about the littermate syndrome and now I'm worried, will sterilize them in 5-6 moths could avoid that? Training then separately? I'm more worried about the aggression than about the codependency, I have other 4 dogs (2 females and 2 males), what I can do to avoid that? Or is not that frequent in littermates raised together with a mom? I'm still debating if keeping the 3 puppies or just 2, any advice would be very helpful!