Getting Them to Understand?

Yes! Especially my 9 yr old son does this too - "I didn't hear you....."
So now when I tell him "It's time to clean up toys and come in for dinner please"
If I don't get an "ok mom" then I ask if he heard me, if he says yes but I dont' think he really did, then I ask "what did I say?"

And yes, you're right, no one can "make" you do anything. You always have a choice.........They should flat out understand that a sibling cannot tell them what they can or cannot do.

I don't think you did anything wrong. Just keep plugging away making them take responsibility for their own behavior and explaining that. they will get it eventually. Have you read "Love and Logic?"

They are trying little buggers.
I say If so and so said jump off a bridge would you do that? If he said eat poison ivy?
No?? THen use your brain and make the right choice.

Keep up the good work. They actually do listen to us, it takes a long time to sink in.

You aren't doing anything wrong! It is just the way kids are. They test boundaries and it takes awhile to learn to take ownership for their actions. Some never learn because someone (usually a parent meaning well) swoops in and rescues them. The best thing I can recommend is Love and Logic by Jim Fay. If you look back at my profile and see how many times I have recommend this book, you would think I am getting a kick back from Jim Fay himself! LOL but I am not. I have just used his strategies as a teacher and parent and it is just bares sharing over and over again. Jim Fay is all about solving problems logically and with empathy so you can keep your cool and keep your child accountable for whatever choice he makes. If he makes the right one-great, but even if he makes the wrong one-even better because it is an opportunity to learn from the mistake! Here is how Jim Fay would have responded to the scenario you mentioned above: Next time give your son a choice "would you like to put your bike away now and play a little while longer in the yard or put it away and come inside?" By giving him these choices, he is given some control of his own, but still within boundaries that are ok with you (the bike gets put away either way.) Since your son decided to follow the advice of his younger sibling, you could say, "Gee, riding around the block again wasn't one of your choices, so now I am going to have to take the bike away for awhile. I am really sorry." Say it with empathy, not sarcasm. Very important! You always want to keep kids in the thinking state. When they get too mad because of revenge or feeling tricked then they get out of the thinking state of mind and we want to keep them there so they can learn from the mistake! If he continues to say it was his brother's fault, then you just say, "well, it really wasn't. He gave you a bad piece of advice and you took it. Now look at where it landed you? No bike. Who is really responsible here?"
It just totally works logically because you are working with natural consequences here. You asked for him to do something (put the bike away), he didn't do it so now he is paying the price of losing the bike. The whole bit about blaming his brother becomes a non-issue. Jim Fay has plenty of books to help you learn these techniques and even offers workshops that I highly recommend too. Good luck to you and I hope this helps!
A.

They all try that. Often.

Just tell them, "Well, YOU are responsible for your own actions." And then repeat the rules.

Educating your kids doesn't happen because you say something once. It's an ongoing process, which hopefully is mostly done when they are 18 and out of the house. Repeating yourself is a normal part of parenting.

They are normal, you are doing fine. Parenting is just hard work.

You've gotten some good answers already :)

I was wondering, though, do the other kids get into a little bit of trouble for being instigators? Obviously, Geiger wasn't in charge, BUT he heard you, and kinda goaded Brody into it. Yes, Brody made the wrong choice and needs the consequences, but Geiger also knew he was doing something wrong by pushing Brody's buttons.

What do other mamas think about this - am I overanalyzing or do the instigators need to get in a little trouble too?

What did you say after the 2nd round of "But Gieger told me I could" ?
If you let it go because you feel bad b/c your son was "trapped" by conflicting things being said to him, then he got away with it. If you responded with something along the lines of: "Gieger is not the parent; I am." then you might start to make a little headway. Following up with making him go to his room is great, but he may not be making the obvious connection about WHY it isn't ok to listen to his brother. Spell it out: HE's not the authority. YOU are.

The bottom line is that YOU (the one with the authority) gave him instructions and he failed to follow them. Just keep repeating that mantra (that it doesn't matter what anyone else said, that YOU are the authority that must be obeyed). Eventually it will sink in. You will give real consequences when they don't follow your instructions. You sound like you might be starting to fall into that trap of arguing with your kids (because they DO sometimes make logical points- or at least you can rationalize how they are seeing it).... but don't.
Read "1-2-3 Magic" to get a better understanding of what I am not explaining very well.