Haven't read the links by Zigner, but can cover this one from the hip. First of all, whether or not the person would be able to draw unemployment would never be legitimately covered in an employment contract of any kind. Unemployment benefits are a federal program, guaranteed by federal and state law and would not be determined or agreed upon or legitimatized by an employment contract, any more than being paid at least minimum wage for work performed would be.
However, those pesky school situations where the person is off during the summer has been covered specifically by the unemployment laws in every state. School systems hate to have to pay unemployment taxes and have forced the system to create some very specific regulations to govern their payment.
If you have "a reasonable expectation" of returning to work at the end of the summer vacation, then no, you are considered a special category for unemployment purposes and won't be approved to draw. Tenured teachers and administrators are usually a definite no go. For non-tenured teachers, usually the school system will tell them at the beginning of the school vacation that they will be reporting back at such and such time. And if they do this, then the employee is not eligible for unemployment benefits. The non-tenured teacher would need to discuss their own special circumstances with the unemployment system. There are different regulations for non-exempt staff such as aides and custodians. But since your sister is an exempt employee, has worked for years, she would not be eligible to draw unemployment benefits during that summer break, unless they had fired her and told her NOT to come back at the end of the break.
But I agree with cbg, there's nobody in the system who wouldn't be happy to explain to her what the law is on this if she cared to call and ask. Or she can do research on the internet.