resources & ideas:
www.flylady.org and there should be a link to the housefairy on this site
Dave Ramsey has a resource for kids that is a magnet chart that you can put on the refrigerator. We make a contract with how much we are paying per chore per day. Laundry (washed, dried, folded and put away) gets a whopping $2.50 a load...big bucks in our household. We write down ahead of time what they are to do. There is also a place for fines and we write down (and sometimes add) items like mouthing off and we take money away if they happen. The big thing with this is that they can see how much they are earning, and its really important to make good on the payment...We like to give payment in full at the end of the week and have them hand back any fines that they owe...gosh it hurts to give it back!
Make sure sister gets paid even if 7 year old doesn't, take the amount that 7 should have gotten and give it to whomever did the chore. Stop nagging, just remind her once at the beginning of the week that you are doing this and take the role as an observer. Give her time to help her understand that this is for real.
Another approach would be to look at the www.nogreaterjoy.org website under child training for ideas.
Good Luck!
Added later: OOps I think it sounds like our 7 year old does laundry usually it rotates through the hands of our 9, 11, 14 year old and me. (mostly the older girls and me though) Our seven year old keeps his room tidy (house fairy has great ideas for this age and younger) helps set or clear the table, take things to the recycling bucket outside and helps unload the dishwasher, some times he dusts (give a boy a tool :) and loves to help me cook, stuffs dirty laundry into the hamper and cleans the hallway...simple tasks and not all are done on the same day.
The important thing is to start instilling a love of working and some how impart a pride in a job well done. Accountability is a nice character trait too, but it will be there if she has the love and pride. Again good luck!