Is this the child's father who is brainwashing the child? If so, that is called "parental alienation syndrome." I experienced this with my older son. I took him to a psychologist who diagnosed this and stated to me the following: "Your husband is engaging in behaviors designed to alienate your son from you." It's subsided somewhat all these many years later (my son is now 19), mainly because my husband drives truck and is gone all week, whereas previously he was driving a local account. It was torture. My relationship with my son has never fully recovered. I wish I had left my husband (I'm still with him and now also have a 5 year old). My belief at the time, however, was "Hold your friends close, and your enemies closer." I was afraid for him to have even more of a negative influence on my son if we split and he had more alone time with him. I'm now 42 and see what I didn't see then - that while I was in defense mode and staying with my husband, my life and the good years passed me by. Children ARE affected by words, and constant repetition by an adult and their influence CAN brainwash even an intelligent child. I would say to nip it in the bud sooner, rather than later. Try to keep your child from this person. Take your child to a psychologist to expose your child to another, healthier viewpoint to counteract the bad advice and brainwashing from this family member. You're a smart mother to recognize that this is going on. Best of luck to you.
C.