Defamation of character
An ex girlfriend went onto my facebook and picked on my friends list my sister in law and sent her messages wrong things about me and telling them how bad and how tgey will suffer and this has now cost y marriage i need to sue the individual
On your facts , it looks like she defamed you and caused you damage. You will need to get a lawyer to sue in the civil court for what you think it is an appropriate amount as compensation. You can also ask for an apology. If she can prove that what she spread was true, it will be a defence.