You're not really going to know how it will affect your aid because different colleges use the CSS Profile information in different ways. The CSS Profile only provides information to colleges, it doesn't determine what they're going to do with it.
In any case, you don't have any option but to explain your family's finances honestly and see how it all falls out, right? I mean, what are you thinking of doing instead? Low-balling what your parents spend for your sister's college? That could make a college you're interested in think they have more money available to pay for yours, right? I mean, there's no way to know how any given college is going to look at it, so.... so just tell the truth.
Colleges know that most parents get seriously squeezed with two in college. They're pretty well-informed about all of this and make whatever adjustments they want to make accordingly.
Normally it lowers what they'll expect for a given student when the parents have another student in college at the same time.