Hmm...
The boys know how to keep things clean, they do help when parents are at home, but are lazy when the cats are away, and don't listen to Sis.
There are two problems here...
The boys
Mom
Let's focus on Mom. I'd sit her down and ask to go through what she expects of you. Specifically, ask her straight out: "If the boys will not clean up, are you expecting that I clean up everything for them?" Oftentimes, people who are holding a ridiculous position realize it once they have to say it out loud. You might continue the conversation... If Mom does expect this, and is willing to say so to your face, follow up: "I understand your position. The boys understand it too, and are taking advantage. Can you help me with this?" Update: It really helps, when you have a problem with someone, to enlist that someone in the solution
Really, I think the principle "Parents want quiet, not justice" applies here, just in terms of wanting a clean house, not equity in splitting the chores.
I'm hoping that putting it out on the table will encourage your mother to dog out the boys -- they deserve it.
Failing that, there are two options:
Train the boys. Tell them you're setting a timer for 9pm (or some specific time) every day; at that time, we all drop everything, and repair the day's damage. Get Mom to endorse this approach to the boys before she & Pop leave.
You're a grown woman with, apparently, dainty feet. Use those feet and stay with friends the next time parents go away. Make sure everyone knows that you are going to do this. Make sure everyone knows that you are going to do this. Come back slightly after parents. Let the boys face the mom-wrath.