Stage make up and foudation?

I'd go without the foundation. Do you have bronzer? Bronzer is really important if you're going to be under harsh lighting, as the lighting will make it appear as if you have no features. Therefore, you need to use bronzer to contour all of your features (cheekbones, jawline, nose, temples). You should also wear some blush to your cheeks so you have colour.

Don't wear the foundation. You won't need it if you use bronzer & blush...

Stage makeup needs to be very exaggerated in order for your feature to stand out on stage. Don't be afraid that it looks garish in the mirror, it's important that your audience is able to see you!

If the foundation is just a shade lighter, I'd still go with it-- but be sure to heavily contour your cheekbones and jawline with bronzer or a blush that has warm, brownish tones.

For the eyes, choose colors with the tone of the character- neutral for a more demure or plain character and brighter, louder colors for more drama. Again, don't go for a natural look-- shadows, highlighter under brow and lots of eyeliner and mascara. Darker your brows with pencil-- you want to significantly darker, but go with a dark taupes or browns if you are blond- you can do much darker/black if you hair is darker. Brow definition is very important!

Line your lips in a slightly darker shade than the lipstick (Wet N Wild 666 is a great multipurpose liner-.99 cents Wet N Wild is the way to go if you need things that you may not use again)

Powder is probably the most import part to set the makeup and help it hold up through sweating under the stage lights. Use a translucent powder or a shade the matches your skin tone. Apply the powder before you do your eye makeup and when touching up your face be sure to avoid coating your mascara. Break a leg!

My suggestion is odd, but it has worked for me in a pinch. Pour a bit of your sisters foundation on a flat non-porous surface. Does your sister have any black eyeshadow? If she does, take a tiny amount(it has to be a matte finish eyeshadow) and add it to the liquid foundation. Then, mix it very very well so there's no shadow fragments in the mix. Then apply it to your face. If it's not dark enough, add a tad bit more shadow. If it's too dark, add a bit more of the foundation till you get the right shade.

You can lighten or darken foundation with white and black. I use products from Face Atelier called Zero Plus and Zero Minus. They can lighten or darken a foundation without undermining the base undertone color of the foundation. One time, I was on set and I needed to darken a foundation and I had forgot to put the Face Atelier products back in my kit. I had to make it work and this is what I did.

You could also just use the foundation and apply a darker face powder that will counter-balance the color of the foundation. Good Luck!