First off in MD anyone under the age of 13 CANNOT be responsible for another child under the age of 8 (sibling or otherwise). Now that said, there aren't babysitting police out there monitoring this. But, I'm not so sure what the consequences would be if there was an emergency that warranted a call to the police/fire depts or a trip to the ER?
Since Min wage just went up to what $7.25 hr. I do think that a good quality babysitter is worth this. But that is my opinion. I know that is why we rarely used a sitter when our kids were young, the cost.
Now many if not most of the teens 16 or up, aren't the least bit interested in babysitting as they want their weekend nights for friends/fun. My 13 (almost 14yo) would love to get some babysitting jobs and she is CPR certified too. She would take what was offered to her and if she felt it wasn't fair/worth it, she just wouldn't go back. for a 2 and 4 yo (depending on if she is feeding them, bathing them, etc..) and how long they will be awake, I would say at least $8 an hour. I also agree w/ the poster who stated though that what is expected should be clearly spelled out (i.e. dishes cleaned up if she is feeding them, toys put away, etc...) I'd also specify what drinks/snacks are acceptable if you choose to go this route. I, personally, took my own years ago. that way, I'd know that I'd like what there was to eat.
best of luck with working all of the details out and I hope you enjoy the concert and your time out together.