She is the sweetest dog and she loves everyone
No, she's not. The sweetest dog in the world doesn't bite her family members. If you can realize that then you're on your way to fixing this extremely serious problem. A dog that bites is dangerous, a 120+ pound dog that bites could kill you.
The question if this is aggression or playing isn't the right question. If she's aggressive then there's a reason for it. If she's playing then she should have been taught very early that mouthing humans is not permitted. Humans cannot play the same way she can with other dogs. What she's displaying is dominance. She thinks she's the alpha female of the home, just under the alpha male, your father. YOU and everyone else, rank below her and she's enforcing that rank.
First, take her to the vet and have them run a thyroid panel and look her over for any physical problems. This kind of behavior is very unusual for a Newfie and highly dangerous. Switch to a very good dog food if she's eating regular store-bought food. This can help with thyroid and autoimmune problems which may cause aggression and discomfort in the dog.
Second, get a muzzle on her until you can get the behaviorist out there. Take a cue from your father. The dog doesn't bite him because she realizes he's the boss. She bites you because she believes she outranks you and the other family members. She's asserting her dominance.
She must immediately learn bite inhibition and the remaining family members, you included, have to learn how to act around a dog. She cannot run the show in that house when your father isn't there. Some things to do right away:
1. Family goes through doors before the dog does. Dog always goes last.
2. Family eats before the dog does. Dog eats last. If she doesn't eat when you do, change her feeding schedule.
3. Dog is not permitted to jump on family members or anyone else. Never.
4. Dog is not permitted on beds or furniture. Get her a nice BIG doggie bed. Those of lowest rank sleep lower.
5. Do not respond to her demands for affection, even if she barks. You decide when to show her affection. You call her to you to pet or play. Ignore her when she wants to play.
6. Make sure she gets enough exercise but don't play tug-of-war games. A good walk is fine for a Newfie but nothing with a lot of jumping as her joints are still developing.
When she does bite you make a big deal of it. Pretend you are very hurt by it. Make a sharp scream and act as though you are in great pain. Dogs use correctional biting among themselves to assert dominance. Her bite wouldn't hurt a dog but she needs to learn humans will not tolerate biting. Then ignore the dog. Walk away from her. Make sure no one else in the home consoles or pays any attention to her for a good ten minutes.
Follow these rules precisely and call a behaviorist! This dog will become very powerful very soon and if she really wants to hurt somebody she will do so. How your family approaches this is very important. She cannot run the house. Do not hit her or call her over to you to punish her or use her crate as punishment when she displays this kind of behavior. She must learn she is the omega, the lowest ranking member of your household. It is up to YOU to do this. Except if there is something physically wrong, she's acting the way YOU taught her to. YOU have to undo YOUR behavior to change hers.
I'm being serious here because this is very serious. She will be huge very soon and as a moody adolescent she needs to be taught how to behave in your family or else she could seriously harm someone without meaning to just because of her sheer size and strength.
Talk to the behaviorist. Newfies are wonderful dogs, indeed the most decorated dogs in the world for valor in battle and heroism in rescuing people at sea. There was a Newfie on
Titanic who jumped into the water and swam the entire time next to a lifeboat full of people. When
Carpathia arrived hours later to find survivors, this Newfie, still in sub-freezing waters, barked long and loud causing the
Carpathia to turn and thus miss hitting a lifeboat full of survivors. The dog was the only animal rescued from
Titanic.
No other breed has given of itself so selflessly as the Newfoundland. Once things in the home are righted you should be blessed with a joyful family member who will give you years of unswerving devotion but everyone in your household MUST understand that she is a dog, not a child, not a human. Your dog will feel much less stressed once the humans are back to running the show. Dogs don't know what spite is and treating her as a dog will make her much happier because it will give her the structure she needs to know how to function.
Think of kids. If every time a kid does something you praise him, he turns into a brat and runs his family ragged. Your dog is acting like Dudley Dursley because, in her eyes, you're treating the way the Dursleys treat Dudley. Dudley couldn't run a puppet show much less a household and neither can your dog. She's acting to lead her family the only way she knows how. Once you step into her shoes she'll stop and it won't bother her a bit. She doesn't want this job but, in dog language, you're telling her to do it.
Good luck and please keep us posted!
P.S. - My signature line was written by Lord Byron upon the death of his dog: a Newfoundland.