I would download fresh copy of Malwarebytes to CD, and use this on her PC. If it is infected, probably best to start it in 'safe mode', then load malwarebytes from the CD, and run it. I did this recently to an infected laptop. It may take some time, mine did. Then shut down as usual, re-start.
You can use remote desktop connection so that you can connect to your niece's computer without having to visit her if she stays far away. The only problem is I do not know if the problems on her computer would then be transferred to your computer. Maybe someone can give you the relevant advice.
Many thanks everyone for responding. Food for thought here.
I know she would not visit dodgy sites, but her ex partner probably did. She uses a BT wireless hub and he connected to the internet via the hub with his Play Station. Is it possible that the infections came via the play station gaming sites?
These days you know longer have to visit "dodgy" sites - many reputable sites are being hacked into and infected with rogue malware programs. Latest reports suggest that a new website is infected every 15 seconds. But it is also true that gaming sites can quite often harbour infections.