Firepatrolchef,
I'm not a pro, but as far as I know you cannot make mayo with whole eggs...but I don't think this is the main problem. Why does your sister have such a limited diet? Is she supposed to lose weight, or has some disease (diabetes, heart disease and so on) requiring a specific diet? If the issue is limiting the caloric intake and/or fats, mayo must be just AVOIDED as it's very caloric and rich in fats. I've never heard about a hypocaloric diet allowing mayo!
In any case, the best way I know to make a light but tasty mayo is mixing it with plain joghurt. I love it even more than the "normal" one.
Hope this helps!
Pongi