when i fight my brothers mario when im luigi.. i cant approch him.. if i wave dash with shield up, he hits me with his forward smash and hits my shield, since luigi has mass traction, i slide way back in my shield. if i jump at him he shield throws me. if i approch with a wavedash down/up smash, he forward smashes me as i approch. in the air is where i can really punish him. but on the ground mario > luigi it seems.. maybe im just bad, but this is usually the turnout.
Don't get discouraged, sometimes that's just the way the matchup shakes out. For example, there is simply no approaching Marth through the air. You would be foolish to keep recklessly jumping towards Marth when he can just swat you out of the air. Don't force yourself to take openings that are not there.
It's true, Luigi doesn't have great approaches on the ground. Anybody that can consistently beat out his D-smash is going to give him a very hard time, and a well-played Mario can do that. I know I've had problems approaching Mario from the ground with Ice Climbers. On the ground, Luigi's true bread-and-butter is punishing lag, not in making approaches.
What you should do, then, is to change your thinking.
Don't just WD up and throw out an attack if he's just standing there. Dance more in front of him and out of range, and try to get him to approach YOU. And when he does, learn to recognize openings; turn the game over to your advantage.
Shield his smashes and WD out of shield to punish. It's tricky because Mario's D-smash is fast and his F-smash is long. I'm not going to pretend this is easy, but it's necessary if you want to take Mario on the ground. I'm almost certain his F-smash can be punished with a wavesmash, or even aerial out of shield, as long as he doesn't sweetspot it. The D-smash may not be punishable out of shield, but I know it has enough lag where you can get inside its range if he whiffs.
If he approaches you through the air...well you already know what to do there. Show him who has the superior air game.
Here's some more concrete advice:
When you do aerial him, space your sex kick and F-air just out of shieldgrab range, or poke him with B-airs instead. It can be done, because Mario's grab range is one of the shorter ones in the game. If you're coming in from directly above, DI behind him to avoid being grabbed. Make SURE you're L-cancelling it correctly and try jabbing or spotdodging right after an aerial.
Try using the tornado more to approach. It's fast and will clank with a lot of attacks. Be ready to follow up on it.
Good luck and remember that Luigi has the advantage in this matchup. Confidence is as important as anything else when playing the game.