Mother boys are extremely easy to gimp when you know what you're doing. All they really have is PKF spacing, and that's all related to whether you can outspace and mindgame Lucas. PKF isn't that hard to get around unless you can't mindgame into close range, because Lucas at close range is "lol". Ness... well Ness is just **** bait. His PKF spacing is just horribly predictable, and the only advantage he has over Lucario is easily spiking out of Bup. Otherwise, he's just a ragdoll for attacks.
Lucas is a different story though. You have to worry about his Bair, because if you aren't prepared to recover around his Bair's spike range, a good Lucas will take advantage of that and abuse it. This isn't just for Lucario; this applies for any character. PKT isn't too much to worry about, but you should always be ready to spot dodge out of most of it's hitbox. If you get hit by the tail, you'll get little knockback, but the head is what could potentially edge guard you. Lucas's smashes can **** Lucario too. Fsmash is unchristianly fast and strong, while Dsmash is generally evil when it comes to edge guarding and spot dodge punishing. As for Usmash, you'll rarely see it unless the player is very adept. Usmash is a great surprise move that Lucas will use to edge guard because the hitbox can hit you on a stage like FD and stage spike. It's not terribly common to see, but it is a pocket surprise move that you should anticipate at high levels of play. Also, if Lucas is approaching, the rare occasion that it is, shield grab or just shield the living piss out of him. Lucas's approaches are terrible, and unless you think he might try to do a Dash to pivot grab, you should always shield him. His SH approaches are laughable and deliciously punishable. SG to Uthrow to SH Fair to Nair is a personal favorite to pull off on Lucas. Lucases almost never approach though, so don't shield crazy unless you are power shield approaching his PKF spacing.
Gimping Lucas is probably the most important idea here, because Lucario can just toss him around in the air. SG/Dash Grab/hell even a pivot grab to Fthrow to Fair is something you aim for if you want to gimp Lucas, especially if he is near the ledge of a stage. If you can, footstool his second jump. All his height comes from his second jump (as does Ness's), so if you footstool him out of his momentum, you can hog the ledge and likely stop him from making it back. Plus, regular BAS gimps (don't rely on these) and Bair are useful in this matchup. Oh, and it's worth mentioning that you could feasibly pull off a DT approach if he is getting predictable with his PKF spacing. It's very risky, as anything with DT is, but it would be great for surprise KOs.
All in all, I'd call it 65-35 adv. Lucario with Ness, and 60-40 adv. Lucario with Lucas. Work it out, the Mother boys aren't that difficult, even at high levels of play. GL son.
edit: lol apparently I've been underestimating Ness, but I don't usually have trouble with him unless I'm having an off day or something.
