New Border Collie growling at family member

Yesterday while my mom was walking home a lost Border Collie started following her, we were concerned for her safety so we brought her inside, that’s when I noticed she had a phone number on her collar; her name is Bridget. I contacted her owner, who mentioned he lets her out every afternoon and initially planned to send a friend to pick her up. However, after an hour of texting, he asked if we wanted to adopt her as it turns out the friend he mentioned was someone he was considering giving Bridget to as he can no longer care for her.

Currently, we're in a delicate situation because we already have a big, unsocial dog in the backyard belonging to my sister. Therefore, Bridget has to stay inside for now, we will be moving to a new house in the next two weeks. My sister and her dog won't be moving to the new house.

Yesterday Bridget interacted only with me, my mom, and my sister at home. This morning, while I was in the living room with her, my grandpa (86 years old) started talking to me and Bridget began growling and then barking at him. She was about to attack him, so I raised my voice and let her out into our front yard (we have a spacious fenced front yard). Now, I'm unsure what to do because my grandpa will be moving in with us to the new house.

Today, we're picking up her leash from her old owner, and I plan to hold her and let her see my grandpa. I might ask my mom to hold her as well (since Bridget is used to her), while I stay close to my grandpa and we can take turns doing that, I'm not sure what else to do 😢. We weren't planning on getting a new dog right now, but Bridget was essentially abandoned and had been hit by a car three months ago. I would feel guilty returning her to her old, irresponsible owner, as she might not be treated well there or by a new family.

Do you have any tips on helping Bridget get used to all family members and understand that my grandpa's presence in the house is normal?

PS.: Bridget is an adolescent at around 2 years old.