Ditto on everything lv4dogs said, plus having siblings can be the worst combo. They seem to fight with each other more than non related dogs because they're always fighting for higherarchy once they get older. You say the dad is an aggressive bulldog, well that can be a trait that he passed on to his pups and the last thing you need is two dogs with unstable personalities that you will have to PTS once it becomes too much of a liability to keep them alive.
Some breeds aren't always a good mix and this sounds like one of those mixes that could be trouble especially with the dad being aggressive. Even if the pups look cute, friendly, happy, that doesn't mean they will always be that way. An unstable temperament won't always be obvious in puppies, it usually pops up once the dog has matured and no amount of socialization or training can cure it.