Personally, I don't like the "no sex" rule. But whether you agree with it or not doesn't factor into this. Bottom line is it's your Mom's house, and her rules. Period...
And the argument that Mom and her husband have sex so why can't we just doesn't fly. We all earn a few rights and priveledges when we grow up, and one of them is making rules to guide our children that we don't have to follow any more!
My girls are 8 and 5 years old (God help me soon), and they are obviously not yet allowed to drink. Does that mean when I have a beer, they should be allowed to as well? If not, does that make me a hypocrite?
I've seen the "mature" word floated around a couple of times in this thread. I think if your sister didn't want this to happen, then the mature thing would be to sit down and discuss with your Mom exactly why she thinks the rule doesn't work. If that doesn't change Mom's mind, then the rule must be adhered to. That's not just the mature thing to do, more importantly that shows your Mom a little respect. I would hope that your Mom had enough respect for your sister to listen to the argument, but that's a topic for another thread...
Many people don't think speeding or smoking pot should be crimes. However, they still have to suffer the consequences if they're caught.