I recommend doing a little research on ADHD. This is as tough for the kid to deal with as it is for parents. Children with ADHD not only have a hard time focusing/remembering things but in their world its kind of a live for the moment thing which is why some kids hurt others and don't realize what they have done. In their mind they have done something and already moved on mentally to something else and have forgotten what was done. Kids with ADHD can also be very good at problem solving which was demonstrated by her having her brothers help her clean her room. She was doing what you said to do and if you didn't give direct instructions that 'she' cleans it without any help, then in her mind her asking for help was ok. Sometimes you have to sit back and try to relate to what a child with ADHD feels. For that kid, it's always about getting into trouble, even when they don't mean to. Things to try are, be specific in what you ask, touch her shoulder or back to get her attention when asking her to do something which will help bring her back to the moment and lastly, try to realize this child just needs extra time and help.
I think researching more on the topic would help everyone in your family...good luck!