hates family member

My fiance has raised Casey our Quaker for 7 years. she is now 8 years old. for the past year since I've come into their lives, Casey has begun to prefer me but was still okay with my fiance holding her or going near her, or even playing with her when I wasn't in sight. For the past 3 months...

My green cheek is 9 months old and he has bonded beautifully with me trusts me completely. He doesn't bite me unless my 5 yr old comes within a foot of us. And he is biting to hurt to draw blood, and to get my son not actually me. (not just the lil nip u get if he doesn't want that quill...