There are still too less information to tell you exactly what is going on. The best would be to let a dog trainer visit you and watching her behaviour towards the friends and your great grandmother. The dog trainer may ask you additional questions and can see if your dog is afraid or aggressive. I have some additional questions for you with possible reasons for the behaviour. You can think about it and may find out, what matches your situation.
Did your dog met your great grandmother before the movement, too? If yes, how was her behaviour then?
When exactly changed the behaviour of your dog? Can you remember the first situation when it happened? What was the exact situation, atmosphere, etc.? Is she really the only person your dog does not seem to like?
How often do the friends and your great grandmother visit you?
Are the friends more like familiy, because they come daily or weekly and your great grandmother just visit you once in a month? Then it could be a territorial behaviour. Directly after the movement the dog didn't feel home in the new house. The friends could come and go, there was no territorial behaviour, bevause it was not her territory, yet. Then she started to feel home and the friends were still ok, because they come frequently. But your great grandmother may be kind of a stranger and so she behaves aggressive.
How is the atmosphere when the friends visit you in comparison to your great grandmother?
Are you more stressed, when your great grandmother is visiting you? Do you like her visits or is it something stressful for you and your familiy? Are there conflicts? Your dog may react to the atmosphere and wants to defend her family.
How do the friends and your great grandmother act and react towards the dog?
Is your great grandmother afraid of dogs? Or does she hate dogs? Does she stare at the dog and avoid contact with her? Or is she completely the opposite and wants to pet and stroke her all the time? Your dog may react to her fear, or your great grandmother harass her and she does not like it. She wants to defend herself.
!!! What to do: !!!
If you are unsure if you can handle your dog, take a dog trainer! The trainer can watch the situation, analyze it and help you to find a solution and training for your dog. Before your dog starts to bite or to fight, take somebody professional.
If you are sure that your dog obeys you, you can try to train it on your own.
Please be honest to yourself! Better taking some help than creating a disaster.
The following steps train your dog to trust and obey you, no matter if she is afraid or territorial. A more indidvidual training would be preferrable, but it is also necessary that your dog obeys you in extreme situations. So this kind of training can help you.
The most important part is that you stay calm and be the boss. Your dog has to be sure that you can handle the "dangerous situation" all on your own and that she does not need to help. If she trusts you that you are able to handle your great grandmother, she will be calmer and more relaxed.
- "Her special place"
If your dog does not already know, you have to train her to go to "her special place", when you tell her. Install "her special place" at a place where she can see everything, but where she can feel sheltered a bit. It should be a place where she is not disturbed and not be able to bite somebody which is passing the way. She have to be able to watch the scenes and be with you, not in a different room. If you change between different rooms, install multiple places for her. "Her special place" should be comfortable. E.g. a blanket and recognizable for her. Train her to go to her place and lay down in a stressless atmosphere. Never use "her special place" as a punishment, but as a place, where she can rest.
- Please do not disturb
Tell everybody, even the people your dog like, not to disturb her at "her special place". No petting, no stroking, no cuddling. She has to be sure that at this place, she is not touched. (If everything is working fine with guests at the end, this rule can be relaxed).
- Please ignore
Tell your great grandmother to ignore the dog if she visits you. It does not matter if she is afraid and therefore she stares at the dog and try to avoid contact or if she loves the dog with all of her heart and always wants to pet her. She has to avoid eye contact and should not talk to the dog. This is not bad for your dog. Your dog does not feel like a human, which may feel neglected when being ignored. She will be more relaxed, because she has not to deal with the guest. But not only your great grandmother has to ignore your dog, you have to do so (most of the time), too. When your great grandmother visits, before you open the door, tell your dog to go to her place, lay down and stay there. Then let your great grandmother in. If your dog stands up and leaves her place, tell her to go back. If she does so, praise her (but not too much, you want a calm atmosphere), and ignore her again. Even if you think your dog is afraid of your great grandmother, don't try to console her with petting and words, because your dog will understand that it is ok and necessary to worry about the guest. Be friendly and calm and optimistic. If your dog starts to growl, ignore her first. If you have the impact that she obsesses about it, say a loud and firm "No" (or a command you teached her to stop). When she stopped, praise her gently and calm and then ignore her again.
It may take some visits until your dog understands that you deal with everything on your own, that she is save at her place and that it is your territory where you welcome everybody you want to. It may be easier if you understand why your dog is behaving like this: fear or aggression. And in case of fear, the reason for the fear. Then you could train more individual and it would be easier. But always keep in mind: Stay calm, relaxed and defend your dog from your guests and your guests from your dog.