What do you think about me cancelling my babysitting job with my sister because she never paid me for the last time that I did it?
Would it put your sister into a difficult situation?
Well yeah, but it's also rude that she never bothered to pay me for last time!
If you made the commitment than you should try to honor it. It is not completely unfair however for you to want to be paid for the day you did babysit.
To be honest, I don't really want to, but I don't want my sister to be sad either.
Try to just focus on the fact that you are being helpful to your sister. Maybe there is a good reason why you haven't been payed. Try to simply get your sister's word for when you can expect your payment.