Thank you for using JustAnswer.
I'm sorry to hear about your situation.
First I will address your second question, about whether you can forbid her from seeing your son.. Yes, you can certainly forbid her from seeing your son, and you will NOT be sued for that. As an aunt, she has no legal right to see you son or take pictures of him, or post those pictures. And as such she has no standing to successfully sue you.
As to your first question, that's a bit more complicated. The extreme situation is where she posts a picture of your son alone. The other extreme is where she posts a picture of a large number of children that your son is in. Basically where your son is the "subject" of the picture, rather than merely a part of the picture, that's where there's arguably a "right to privacy". Now to be entirely honest, this is an "emerging" area of law. Social networks are still relatively new, as far as the legal system is concerned (the legal system moves at a snails pace in a race-car world) and there's very little caselaw regarding this. But as the guardian of your son, legally speaking you have a right to control his image, and to the extent that he's the subject of a picture (the main reason for the image being made) you can control it. But if he's merely one of many in the image, then he would be incidental (much the same as someone taking a picture on a sidewalk where you just happen to be walking by at the moment).
Hope that clears things up a bit. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for my answer unless and until you rate it a 3, 4, 5 (good or better). Thank you, ***** ***** luck to you!