What color is your sisters hair?
A dark background using one flash for a rim/hair
light might work better.
I don't know that one flash would be enough to blow the background out to white.
Use a reflector for fill (a large piece of white card or a windscreen reflector will work if you don't have one in your kit)
Remember to keep a good distance from the background and if your
lens is
sharp wide open work with an
f/stop starting from around f4 and up to f8. Play with them and see what you like.
Remember to focus on the eye closest to you so single point focusing is best.
Have fun.