Hey habosh,
Wow you're a really nice sister, teaching your brother! Can I ask, why do you need to teach him? That's not a criticism, just a genuine interest! I could never teach my brother when he as younger, he wouldn't listen!
Ayway, to answer your question, or at least attempt to, there are many different teaching methods (I'm sure you're aware of) and some work for others, whilst some don't help at all. See, as you say your brother moves around a lot, he may be more of a 'doing' learner as opposed to a 'study' learner. He might find things stay in his long term memory if he actually 'does' them. Some people can't sit and read books, and retain all the info. Ther are other types of learner, such as 'visual' which would obviously include pictures, and depending on the 'type' of learner you are, you will learn better from different methods. If you two aren't the same type of learner it might cause a bit of friction, as it seems to have, but a compromise will overcome this. For example..'You read about this battle then we'll act it out' just so you get a bit of both in.
As for you being a bit disappointed with some of his grades, I'm not sure. Maybe let him know he's done well but 'lets beat that next time' just so he won't feel bad for not getting the top marks. Then if he does better next time you'll take him for a treat...his choice (within reason if you decide before hand!) could be from a McDonalds to a new T Shirt! Whatever he'd like!
Finally, this is probably obvious, but let him know you're still his sister and you love him for being him, not just for getting an 'A' grade. Have fun with him, without incorporating study, and just have times when you chill (I'm sure you both do!) It will help!
I just have to say once again, 'Well Done' for being so fab! You must really care about your brother to put in all this time and effort!
I'm sure the rewards will be more than enough for this!