Dogs learned to be reactive from another dog

Hi! I’m seeking advice on how to unteach negative allelomimetic behaviors. My family’s two female dogs have previously been friendly with other dogs and people. Another family member’s male dog, whom is reactive towards strangers and other males, visits the house most weekends. He seems to have fear aggression as well as same-sex and territorial aggression. However he adores our family and close friends that are in his “circle of trust”, as well as female dogs and even most small male dogs. He gets along well with both of the female dogs in our family. We’ve had the females for a few years now, and the male has always visited on this consistent schedule.

The three dogs all get overly excited when someone comes to the door, enters the yard, or walks past the house. They’d probably all be barkers to an extent if they were alone, but they definitely seem to rile each other up and have a pack mentality when they’re together. Recently, the two female dogs have escalated from just barking at strangers to more aggressive behaviors (growling, snapping, etc.). These actions seemed to be learned behaviors from the reactive male, as this is how he behaves when encountering strangers.

It’s a really upsetting situation. One reactive dog has been a lot to manage for the family, but three is beyond overwhelming and an even bigger liability. It’s disappointing because the female dogs were always very social with even new people and other dogs. It seems to be a new development that they are mirroring the reactive male’s negative behavior, despite the three of them being very close for years. Even when he’s not around, they appear to be reactive in the yard and leash reactive (usually just barking but occasionally growling at unfamiliar dogs) on walks.

The females are still typically well behaved at the groomers and the vet. They love playing with their dog friends, especially the other regulars at their weekly daycare. They’re friendly when people they know come over, although they still bark excessively in a happy greeting. The older female has a very docile and gentle personality, so we are especially shocked by her mimicking the reactive behavior. The younger female is a cattle dog mix so naturally she is a bossy busybody, but we never thought she’d behave in an outright aggressive manner. The reactivity right now seems to be limited to being territorial over their yard/house with strangers and leash reactivity towards dogs on walks.

The male dog has received a lot of training to manage his reactivity, and he is always kept at a distance from his triggers for everyone’s safety. My family is planning on seeking help from a trainer for the female dogs as well. Additionally, we intend to do solo walks for the female dogs and slowly counter condition them to triggering stimuli. What else can we do to discourage the negative behaviors that have been unintentionally modeled by the reactive dog? There seems to be limited resources on how to stop a reactive dog’s anxieties from rubbing off on other previously friendly dogs.