you don't do stuff like that for money. you do it because you think it will be fun.
the likelihood of getting paid was but one of my qualms with the project, and it probably wouldn't have affected my decision if the project didn't look like a festering, steaming pile of shit.
however, i think you DO do things in part because you're going to be paid, particularly when there are other people who could potentially make money from your work. i know sometimes people get all juiced up about the idea of participating in something like this, and they forget about the fact that it's all business. unless you simply feel like being helpful because it's a project your sister is putting together, i think you've GOT to get paid.