Mhhh...Well, I'm not SUPPOSED to do this because I play Maxima and I don't want Maxima to lose, but I wouldn't want you to quit someone you like.
Lemme cut you some slack here.
If you're matched up, Iori vs. Maxima, and your brother continues to throw out Vapor Cannon (C version), or far C or far D (if he does far D, immediately do what I say). Get in range of the move. Just close enough for Maxima to hit you. This time, wait for him to do something. If he does anything of the above, roll through it as slowly as possible. Maxima has a huge cooldown when he does those moves. If you roll through them, you're going to get in for a full combo.
Also, Maxima only has one move that guardpoints lows, d.C. He is extremely vulnerable to lows. He can DP+BD (EX), but if he misses that, you get a full combo. Your st.B is a low. Mix that in with d.B's and make him commit to blocking low. Once you do that, if he's committed to d.C, he'll try to do it to guard point. If you empty jump to scare him into doing it or just do it late so he mistimes his d.C, you get in.
You should also empty hop against Maxima. From close range, jumping in and attacking means he's going to guardpoint you. If you always attack, he'll always guard point. So don't let him by just landing. Iori has a 1f command grab. Grab him the moment you land and punish him lots with some follow ups.
You can also cheat guard point. You can grab Maxima in the middle of guard point if you did an A move. That has very little recovery so it allows you to grab him or do another move. Aside from st.C close and far D, all his other moves only guard point once. So you can poke him with A's or poke then regular/command grab him out of it.
Maxima also has very little ways to deal with people who are crossing him up. His st.A is awesome, but it's one of the slowest in the game. Iori has a j. back+B crossup that can even hit crouching people. Jump over your brother's Maxima and do the command crossup!
Try it out. I think if you wait for Maxima to do something not safe, Iori can get in big damage so don't quit just because of a bad match-up!