Just in case you can't get back to the thread right away.
Normally the contact arrangements would continue. So if they are taking place in a contact centre, that must continue, because they are Court ordered.
Your Mum will be told to phone the Police, if the Parents turn up at the house. She must not open the door or engage.
If your Mum is approached at the School/Street, she must not engage and if not left alone, must phone the Police.
That way an order will be made by the Court and it becomes an arrestable offence.
The SW should lay out your Mum's Safeguarding duties and contact with the Parents will be included.
If your Mum can't safeguard them, she will have them removed.
If the placement goes well, your Mum can apply for an order, similar to an adoption and contact may stop, other than letters with the Parents.
Your Mum will have to engage with SS, attend meetings, or give a written statement, if ever needed.
Is it just one Niece that she's taking? She may have to follow a judges order for Sibling contact, which can just be a meet up at a park, to suit everyone.