Allistair, I've been in the same situations as you.
But, don't worry. If you have good friends that have known you for a while, they'll have no problem distinguishing the you that they've always known, from the toxic ramblings of someone that tried to steal you away from them! c:
And even newer friends, or people that have never met you before will likely take such words with a grain of salt, and judge you by theirselfes if they meet you in person.
The only advice I can give you is to not chase this situation too desparately, and to just blatantly ignore anything that your ex is doing, and wave it off as "just another crazy ex' thing" to anyone that asks.
The last thing you want to do in such a situation is give her the feeling that whatever she's doing is having some effect on you.
@Emtile
I can't imagine how bad you must be feeling now, but try to not let this get to you too much.
The only thing I can say about this, is that if your uncle was a nice person, and loved you as a family member, he would only want you to be happy and succesfull, despite his passing.
And, don't be affraid to talk to anyone about this situation.
Mourning over a passed family member is a completely normal thing, and young people are generally known to have a little more trouble with it. So, undoubtedly, there will be plenty of people in your family, or on your school / college, that you can ask for help, regarding this matter.
I hope you don't let these obstacles get the better of you!