Congratulations, and welcome to the forum!
When I was a teenager, my mom had a standing rule that we could always use her as an excuse...'Mom just paged (hey, it was the early 90s!) and I need to go home now' or 'my Mom would kill me if I did that,' even if it weren't entirely true. In general, we were expected to stand up for our own principles and be honest, but she considered it an important tool for us to have to get out of uncomfortable situations on short notice.
Now I work in the financial world, and we tell our clients to always feel free to make us the bad guy. Depending on the dynamics with your sister, this may be the best direction for you to take as well.