I cant think of a reason why it shouldn't work for you.
If it came with a set up disc/instruction manual it would be best to follow that.
If the router is wireless then you should be able to connect wirelessly. If she set up passwords on the router and cant remember them then you may have a problem in that area.
Doubt very much that they will ask for it back if your sisters contract ran its course and the router is a year or so old.