MelissaRB, it's super difficult to lift your hair evenly with just box dye if your hair is dark... your hair looks around a 3 or 4 brown (but looks lighter in the second photo you posted, maybe a 5?) and the picture you posted is around a level 7 golden blonde, I'd say? Just dyeing your hair with box dye once won't damage it too much, so it's worth a shot just trying the shade you wanted (the champagne blonde you wanted) to see what shade it gets your hair to -- i can't see that being disastrous, i think the worst case scenario there is that it's just not as light as you want, since lightening with just box dye is nearly impossible. no box dye that doesn't claim to have a "bleaching kit" is going to lighten your hair more than a shade.
If you really want to go lighter without going to a salon and if your hair is its natural colour now, why not try some lightening shampoos/some of the tricks with lemon juice etc folks have posted in the past here to see if you can lift it a bit? Foils are also quite cheap at a salon if you only get a few, and even just a few honey ones or a bit of balayage around your face might lift the overall look enough to read as "blonde" like you want.
If you REALLY want to lighten at home and your sister is handy with hair like you said, you could break the base with a soap cap (mix bleach, developer, and conditioner or shampoo -- it's a weak bleach that lifts hair just a bit) or a little bit of bleach with very weak developer (20) to lift your whole head a shade or two, let it rest a day or two (you'll probably be a bit splotchy, get a cute hat) and then dye over it with around a level 7 blonde. That could get icky real fast though even if you do a good job! There's just no way to tell what your hair will do
i've also always dyed my blonde ashier than i want since it tends to get brassy quickly, especially box dye? you may want to avoid box dye that is too "golden" as it really may come out more "orange," especially if you've done any pre-bleaching.
also FYI -- looking at the clairol site and their numbering system doesn't seem to quite line up with the standard, so just compare their charts to the chart dauphine posted above! at least in the US the chart she posted is a pretty standard numbering system for haircolours.
OMG SORRY THAT GOT SO LONG