First Non-relative Adult Babysitter

First of all, you need to prepare yourself! If you are letting her watch them, then she needs to do just that. No mom stepping in all time or checking on them every 15 mins. I made that mistake and insulted a family member. She thought I was implyng that she didn't know how to watch my child. That said, make up a list of very specific house rules, including what to do if one of your children breaks the rules. If she will be there while the kids need lunch or a snack, make sure you set it out ahead of time and let her know exactly what your expectations are. This situation will work best if you are in another room or out of sight. Young sitters especially, will be really timid about disciplining your kids if you are right there. They sort of expect you to do it, I think. Give it a shot for an afternoon or for a few hours. If it works..great! If not, well nothing lost!

I would train them to babysit just as if you were not there but set clear lines as to when you want to be called in. For example, if the boys are arguing over a toy, take it away and tell them that they each need to pick a different toy to play with, if the boys are actually fighting come get me. At 11 the girls should be able to use their own judgement about simple things like can we watch tv now, or can we have a popsicle if you've made the kids routine and basic rules clear. If the boys see that she is coming to you about everything they will make sure that you are interrupted as much as if she weren't there. Plus even though the girls are young now, this is a perfect opportunity to train them in the right way to watch YOUR kids, which may come in handy when they are older and grandma can watch the kids on date night.

My cousin has a young lady who comes in once a week while she runs a quick errand or pays the bills - things that she cant get done with two young angels around - I believe that although their mom or dad is around the house somewhere the children know the rules and understand that the "mom's helper" expects the same rules to be followed and doesnt seem to have a problem with them following and staying in line so to speak.
I had a neighbor that offered to do that for me and really it isn't to discipline them but to help keep them occupied while you are busy with a project that would be faster completed without "extra" help.

11 year olds will do what ever you tell them to do-- they aren't old enough to think they know everything yet :) so she won't be put off if you give her a list of rules or tell her what you expect.

mostly encourage her to play actively with them... and come get you if it's something urgent.

If they are only 11 and all you are looking for is a mother's helper not an actual sitter right now, I'd say of course go over the rules. you don't want her running to you every time the kids want to play a new game or go to a different room to play, and you don't want her just taking off with them to the park without you knowing. set firm boundaries about snacks, and where they can and cannot be and can and cannot do. As for discipline 11 is to young to effectively discipline another child. leave it at she can tell them no, or to share, or be nice but for anything more serious than that she needs to get you.

i wouldn't worry to much though, your kids are going to know you're still home, and probably won't push the limits to much, and it's only ever going to be fore a couple of hours at a time.