If they have only laid eyes on one another recently, it's normal for the older ones, who have been around longer, to chase and bully because the younger ones are strangers and intruders in their eyes.
Ideally, when you get new chicks, raising them alongside the older chickens helps them to become members of the flock by proximity, and when they become old enough to mingle, there is very little conflict because they've long been accepted as belonging.
Introducing chicks only after they've become fully grown produces more conflict because they are the same size and are looked on as threats to the existing social order while small chicks are not.
If the Brahmas have only been mingling with the older two for a week, that's not long enough for strange new chickens to become accepted. It took a month for a hen I recently introduced to my flock to be fully accepted. The older ones need time to get used to the new ones, while the new ones need time to gain self confidence with the established hens.
Do what you can to help the Brahmas build self confidence by giving them a safe space during the day where they won't be bullied during the next three weeks while everyone is getting used to being together. After a few weeks, it should be safe to let them mingle outside.