The chances of them fighting are equally high if they’re litter mates. I would advise you to get a neutered boar (which you should be able to from a shelter) and a few sows if you really want a group. Is there any reason you specifically want boys?
I am the owner of two boys myself and they’re perfectly happy being together and I’ve never had an issue with fighting, but for most people things can be tense during the teenage months, it’s also all about character compatibility. I have no experience with sows so I wouldn’t know any better.
Usually the advice is two boys, or a boy and group of girls, or of course a group of girls. I of course have only experience with boys and even then have never had any issues with fighting. My current boys Smokey and Bacon both get along very well