OK.
So she has no sponsor then?
In which case she will need to demonstrate that she can fund the travel, accomodation and touristy things herself during her entire stay.
If she will provide an itinerary I guess she can mention her sister and also her good friend Nikki
Seriously Nick, I think she can only do what she can do, unless you want to be co-sponsor for accomodation, then of course you need to provide passport page copies, bank statements and proof of accomodation for her.