You need to do the best job you can for your sis or she'll tell mom on you.
Assuming you have attic access overhead:
First I'd check out the proposed switch location and sound out (knuckle rapping) any possible fire block or mid-span block that might be in the way of fishing the wire down from above. If found, either cut an opening for a 2-gang old work box immediately above or 2" below it (allows top room for cable entry). That way you can avoid the blockage or drill it easily for passage. Fishing the wall is always fun... I favor using a piece of drop ceiling hanger wire but there are any number of things you can use depending on what you have and if there's insulation in the wall (no likely this time).
Then I'd fish a 12/2 NM and a 12/3 NM down to that box... the 12/2 running to the power source and the 12/3 running to the ceiling fixture box.2-gang old work box at the switch location.
Then mount an octagonal box to support the fan either using a spanner bracket or 2x4 between ceiling joists and four 1-1/4" screws.
That's all except for doing the shopping for parts. You'll need a couple or more of 3/8" 2-screw connectors, a 2-gang old work box, an octagonal box, a couple of switches, a 2 gang switch plate, a few orange or yellow wire nuts, some cable clamps and the 12/2 and 12/3 romex cable (or 14 ga if you decide to go 15 amp) of the proper length... little longer to be sure.
When finished, collect a hug and that 12-pack she promised you!
Now if there is no attic above, you might consider talking her in to moving to a house with one. :smile:
ps. If you see a some source of power in the attic short of having to run all the way back to the panel, I'd not hesitate to put this small load on nearly any receptacle circuit. Might be a lot easier. Be careful and certain that the power is off before working with any existing wires up there. You can cut a cable serving a wall receptacle and set a j-box for splicing the new run into it if you can get enough slack... if not enough slack, you can set 2 j-boxes and still get it done.