I can relate to you, 100%. I have been there, and there is only one thing for you to do--get away from that woman asap.
One day I will write about my story. But for now, I'll just tell you the short version. I was friends with a girl in high school, and a few years later, our parents met up and actually got together. I lived with them, and so she eventually became my step sister.
When I got pregnant, she was very supportive. After my son was born, she would help out a lot. She would babysit for me, and buy him lots of toys.
And then, over the years, she became obsessed with my son. It was all very gradual, and I did not see. I just thought she loved him, like any good aunt would.
When I moved to NC, she was very upset. She would keep pressuring me to sign a custody letter, basically giving her custody of my son in case something happened to me and my husband. She bugged me so much about it that my husband and I would secretly joke about her "whacking us", or cutting the brake lines, etc. I mean, what lunatic would harass their friend about signing their child over, "in case they die"?
Things got worse and worse with her. She was so possessive over my son. When I would visit, she acted like she was his mom, and I was just the "egg donor". She could have cared less if I were there, and actually wanted my son all to herself. She would offer to babysit every time I was there, just so she could have my son. Being vague, or wishy washy is NOT the way to deal with this type of person. Avoiding calls and emails is not right either. They will not get the hint until you tell them to leave you alone. That's what I had to do, when my son was almost 6 and I couldn't stand things anymore.
Please don't let her around you guys anymore. I wish, wish, wish I could have "dumped" my friend a long time ago. Of course, since our parents are married, it was really hard to do this. But ck1, please take my advice. Please get away from your friend right now. I never thought this kind of thing could happen to me, and if I can help warn somebody else, then I feel I have not gone through this experience in vain.