I am a 21-year-old woman who still lives at home while attending college. My parents have three younger children between 2 and 9.
While I don't mind watching the kids for my folks when they go out, they seem to think I am obligated to, and they put a lot of pressure on me to change my plans to baby-sit.
Recently, I have had problems with the children being disrespectful and not listening when I ask them to do something. It becomes difficult when my mother ignores their bad behavior.
If the kids bug her enough, she will give in to them, even if my father or I have said no. They tear up the house, and instead of making them clean the mess, she just does it herself while they play.
She tells me her parenting style is her own business, and to stop trying to turn her children into "soldiers" with all my rules. But if she expects me to watch her children on a regular basis, it is my business.
You and your folks should sit down together and work out a baby-sitting schedule; your parents should agree to respect the schedule, and you should agree to be available.
You should explain to your parents that when they don't introduce some limits and consequences with the younger kids, they limit your ability to take good care of them while they're with you. This affects your authority with them - and it is a very important safety issue.