To balance the exposure between ambient light and flash light is quite simple:
Ambient light is controlled by your shutter speed, flash/strobe light is controlled by your aperture.
In the scenario you have quoted above, it will be very difficult for you to overpower the ambient light with a single Sb600 in the middle of the day, what you could have done is either find an area with less ambient light (under a bridge, in a woodland in a building) or waited until later in the day when the light is much lower.
Then setting the aperture you require for a correct exposure when lit by the flash you can then control the amount of ambient light by altering the shutter speed from a slowish shutter speed of say 1/30th sec (the actual flash will be much quicker than this and will prevent subject movement from showing) to the maximum sync speed of your Camera.
You can also introduce some creative effects by slowing the shutter speed down even more and getting your sister to 'do a twirl' with you tripping the shutter at an appropriate time, the flash will freeze your sister whilst the ambient light will allow some movement to show in her dress, this will also pehaps lift your images into something a little more expressive than those shown above.