Rescue parrot dislikes rest of my family

We recently adopted a Quaker Parrot. Her name is Polly and she has biting issues, and is a shoulder hugger. She seems to think biting people is fun due to their reactions, and she opens her mouth and just kinda hits you with her beak, along with hissing. She knows me and our family friend don’t put up with biting and she seems to prefer us so she doesn’t try to get us. But she’s suppose to be my 85 year old grandmothers friend. She got into her shoulder and bit her really hard. My grandma is afraid of her and any time I try to get her to step up onto my grandma she starts rasing her feathers and opening her beak to bite. She’ll sit by her and mess with her hair but will attack if my grandma gets to close. Please help. She was suppose to my grandma’s last bird, and she really seems to have the potential to change. My grandma likes her just not the fact that she bites. We aren’t in a postion to be able to afford a bird trainer right now. Any help or tips would be wonderful. We don’t want to give up on her.