Welcome to the forums, Noah C!
My friends and I used to listen to (and play) the same kind of music you're talking about. Back in the day we thought those bands rocked and other bands were lame. I've become more accepting of different music since then. But I still enjoy that music, too.
As far as originals go, well, yes, you can out grow them. I seem to remember reading that Geddy Lee from Rush doesn't like to play their earlier songs anymore for pretty much that reason. He feels his songwriting is much better today than it was all those years ago. Of course the fans still like all of the old material.
As a songwriter myself I know that some of my songs have evolved over time. Maybe you can re-write the parts you don't like? Other than the whole unplugged thing most big name bands don't change their songs too much once their recorded because they don't want to alienate their audiences. At the local level that's probably not as big a deal. So, especially if you're just talking about two guitar parts, I'd say go ahead and re-write them.
You're going to have to figure out a way to communicate better with your brother. It'll only strengthen your relationship in the long run. It's even more important in your situation since you're both in a band together. There will always be conflicts; you have to find a way to resolve those conflicts in an agreeable manner.
"Listen, I know you really like that song we wrote a year or two ago. I thought it was pretty cool back then, too. However, I think I've grown as a songwriter/guitarist since then and there's just a couple things I want to change before I'd feel comfortable performing it again. I think it's important to make these changes so our song will fit better with the other tunes we're playing now. I just want it to rock out a little more so our audiences will dig it more. So tell me what you think of this ..."
BTW since your brother is the lead singer, he probably "loves" that song either for the lyrics or the melody (or both). He's probably less attached to the guitar part(s). As long as you're not changing his words or melody he'll probably be willing to go with the changes. Otherwise you may have to settle for a compromise.