I think Sheryl T has the right approach. I have a four year old also, but it seems that these teens need a firm hand to develop some good habits now, before it is too late and they are on their own.
After all, if they choose to live like that once they are on their own, at least she doesn't have to see it (it isn't her house), and they will clean up their act when they see how their peers live.
The teens shouldn't be asked to 'try harder' to do their laundry weekly, they should be denied going out for fun stuff if they don't meet expectations. No laundry done? No going out with friends to hang out on weekends, or pull whatever thing they like away from them - no movie rentals, no computer time except for homework, no video games, no phone time (no cell phone if they have it) etc. It's unfortunate, but probably a fact of life with teens.
I like the vinyl sheet idea, but I had an uncle who was in the Army who was pretty strict line - no makee the bed, no sleepee the bed. He would wake them up and roust them out of bed if it wasn't made - only took once or twice sleeping on the floor before my cousins caught the hint. I would probably do that, rather than the expense of buying anything.
Just like with 4-year olds, consistency is key in applying whatever punishment she lets them know (in advance) will be the result of not meeting expectations. "It's Monday now, and if your laundry isn't done and your bed made up by Friday, you won't be going out Friday night, or until it *is* done." Then, come Friday evening, she's got to stick to her guns and not let them go out with friends or whatever until the laundry is done and the bed is made.
Oh, and as for the complaints that other kids their age...blah blah blah - just tune it out, it's the age-old whine about what someone else's mommy lets *them* do! and should be ignored or countered with the standard my house, my rules, don't like it, too bad, (OR go live with your messy friends OR earn some money and hire a maid) mantra.
Good luck!