My ex's partner was married into my new partners family but they split up and she got together with my ex, She was always known as aunty to my children. All ao this plus her messy divorce has resulted in a huge fallout. Iv'e had her follow me in the car, abusive posts on websights plus her family have joined in now. I have made it clear I don't want her anywhere near my child but now they are starting to turn up whenever my child is at his grandparents. Is there anything I can do to stop this?

Probably not, but you need to say where you live as laws vary by state. Also please say what the court ordered custody and visitation arrangements are, if any, and try to say who is who a little more clearly.

Try checking out stalking laws in your area. You may be able to get a restraining order.

... and try to say who is who a little more clearly.

She apparently used to have a [domestic relationship short of marriage] with the person with whom the OP now has a [domestic relationship short of marriage]. Additionally, and in the past, she was married to some relative of the person with whom the OP now has a [domestic relationship short of marriage]. What that previous relationship may have been is not, in my humble opinion, significant, other than a partial explanation of why she is now called "Aunty".

Of course, "ex partner" is shorter, but not as precise. When I see the word "partner" used in this sense, I'm sorry, but for a milisecond I still think "business partner" (the relationship Scrooge had with Marley, for example).

The wisest men that e're you ken
Have never deemed it treason
To rest a bit and jest a bit
And gather up their reason

I just read my post back and it doesn't even make any sense to me... sorry guys I'll try to explain it a bit clearer. The lady in question used to be a very good friend of mine and had been so since we were 3 years old. I had a baby with my now ex boyfriend and went on to be married to another man and have 2 more kids. My "friend" was also married to my husbands brother but they split up having a rather messy divorce. Hence why my kids call her aunty. She has now started a relationship with the father of my first child but does not want to see any of my kids except this 1. I have had her banging on my door 2 weeks after giving birth frightening the child in question, also had her drive very close behind me reving her engine while the child was in the car as well. I live in the united kingdom.

Sounds a little like my past, except I never had kids.
Sooner or later your ex may try to get a different custody arrangement at her insistence, so be prepared. And document her harassment, and possibly get a restraining order, keeping in mind that it might speed up any court battle between you and your ex.
Don't forget that saying my child rather than our child may fan the flames too. Is there no visitation at all?

This "friend" has no legal standing. Her only relationship to any of the children is by marriage.

I think the best recourse for you is to try and get a restraining order against her and/or prosecute her for stalking.

Yes His access is every Wednesday after school and every Friday as well, These are all supervised at his parents house. I have nothing in wrighting yet but I am in the process of seeing a lawyer to get the ball rolling. It really effects my child every time he sees her his behaviour gets out of control (he has Autism) and as always its me that's left to calm him down and reassure him everythings OK :-(