Ask an Property Solicitor. Get an Answer ASAP.
I am a solicitor.
I would advise you to get a local solicitor to write to her children and point out the correct boundary line and asking them to remove the wall from your land or you may do it at their cost.
May I help further?
No, that is not always the case.
You may get a boundary expert to determine your boundary line for you.
All the best