Since your brother has apparently turned to evil, it's time to set foot in the fortress he built, and bring him to justice.
Like Mega Man 4, the first stage of the first fortress is as far as the passwords will take you. Once you've beaten all eight robot masters and you get all the Beat letters, write down the password. It's the best one you can get. Remember that passwords will NOT save Energy Tanks.
This isn't too difficult a stage, but there are a few portions of the level that will make you want to tear your hair out. My advice to you, though, is just hang in there.
The tigers from Napalm Man's stage reappear, so in the first section, be sure to have a fully-charged Mega Buster shot ready.
Also reappearing is the walker robot from the Star Man level. Just beat this guy the same way you did before - shoot him in the eye repeatedly.
And, of course, what Mega Man game would be complete without an "appearing and disappearing blocks" section? These are above a bed of spikes so they're particularly annoying. But if you use Rush and the Super Arrow effectively, it can cut the difficulty here greatly.
Waiting at the end of the first stage is the first Dark Man. This gray robot patrols the room on treads, firing at you as he does so. With each hit he gets faster, so be careful. Water Wave is his weakness, and since he's on treads, he can't jump over it. Just Water Wave him to death while dodging his attacks. This guy isn't too tough.