i personally think it could have worked
because at the time of Co Roxas was still learning the basics of fighting and so was xion so those were 2 useless members next up comes Luxord Sora was able to take Luxord himself and i remember Marluxia being a rather powerful foe i feel that it would be like this
Sora vs Luxord Win
Larxene vs Demyx Win due to her magic countering his water
Marluxia vs Axel Pretty even but i'd say loss Refight
Marluxia And Sora Vs Axel Win
next up is the 1 i think they would have had trouble with Saix Appears saying he will not let them go any further while roxas who is weaker than sora beat saix and sora beat saix sora vs Saix = win
Zexion was already eliminated by the time the plan would have come into motion but if he wasn't he really wasn't very powerful i'd say larxene vs zexion Zexion loses
Lexaeus Vs Marluxia I'd Say Marluxia would Make Swift work of him with his specter because lexaeus was 1 of the first to fall to riku
Vexen and the riku replica vexen was weak the ice scholar will fall to marluxia and sora because marluxia fights vexen while sora takes on the riku replica Sora uses starts out with a flurry of attacks using strike raid back to back all missing riku replica uses chain rave as sora dodges he unleashes a blizzaga which hits repliku as repliku counters with dark firaga sora blocks and unleashes a triple blizzaga followed by thundaga as repliku blocks it all the scene changes to marluxia revealing his specter he uses his twins scythes and sends an energy blast which breaks vexens shield then sends a flurry of his flowers to destroy vexen and vexen dissapeared into thin air as he must of faded into darkness
the scene cuts back to sora vs repliku repliku is on his knee and he uses his blade to help himself back up and the fight commences once more sora using chain rave and hitting repliku dead on multiple times with repliku's unbreaking will he stands up again to sora's shock sora uses strike raid and follows up with a firaga burst fatally wounding repliku as repliku fades away he apologizes to vexen for failing
Next up is Xaldin he will be much harder from the data i've collected it would take Marluxia and sora if he uses all his assets being the dragoons and his lances Marluxia if he went to his second form the grim reaper and Sora as the mindless puppet he is would strike at marluxia's will as a decoy and marluxia would appear out of no where [i kinda feel nobody gives marluxia enough credit due to the card format he wasn't really good in com] as sora distracts Xaldin Marluxia swoops in for the kill with the grim reapers flurry of meteors
As Xaldin Dodges marluxia appears next to him slashing him with his grim reaper scythes wounding him as he jumps back sora uses a firaga on him which misses but marluxia unleashes another flurry of meteors on the wounded xaldin xaldin manages to block it but takes some damage and collapses to the ground as he tries to get up marluxia unleashes an painful flurry of attacks with no mercy Next is Xigbar i can see him giving marluxia a run for the money and sora coming in with a Firaga attack and marluxia getting out of the way afterwards sora launches a flurry of attacks with chain rave as xigbar being his nimble self avoids it marluxia uses a energy blasts from his scythes in the shape of an x and xigbar dodges once again unleashing a flurry of energy shots as they seemingly don't affect marluxia marluxia goes to xigbar and slashes at his arm braig snapping out of his fear induced paralysis moves out of the way only for marluxia to launch a flurry of meteors hitting him point blank thus destroying him next is xemnas marluxia sora and larxene try as hard as they can none of their attacks seem to be breaking through next thing you know a sudden light appears to sora and the memories start flooding back he uses the power he gains from his belief in his friends and he attacks xemnas head on with marluxia going to angel of death sends a flower blast at xemnas and sora using firaga burst seems to wound xemnas but xemnas counter attacks with his force lightning rip off attack knocking marluxia to the ground as larxene finally stands up going to xemnas and casting thundaga and he deflects it back at her and fatally wounds her as she is fading into darkness and dying she unleashes a final attack and puts all her energy into 1 of her throwing knives injuring xemnas as xemnas throws a building at sora sora cuts through it and blasts him with another firaga burst as marluxia uses another blast of energy in an attempt to finish xemnas but he quickly deflects it still having so much power to the shock of sora and marluxia they combine their efforts to finish xemnas with 1 last attack sora using strike raid as a distraction to lead him into marluxias energy blast as xemnas fades sora turns his sights to marluxia but marluxia summons namine and asks if he's willing to strike through her and he drops his keyblade then he has her rearrange sora's memories once again
Roxas and Xion i will not put in here because i can't really think of how it would turn out my imagination died when thinking of how those would be if someone else wants to add on then they may do so
This theory hinges on Marluxia, Larxene and Sora taking the Organization members down one at a time. What if all of the other members confronted the 3 of them at the same time, and they were forced into fighting all of the other 8 members at once? There's no way Marluxia, Larxene and Sora could defeat Xemnas, Xigbar, Xaldin, Saix, Demyx, Luxord, Roxas and Xion all at the same time. And that's assuming they managed to defeat Vexen, Lexeaus, Zexion, Axel and Repliku first--which is more probable but still unlikely.
I think it would've made more sense for Marluxia and Larxene to attempt to convince other members of the Organization to side with them. They easily could've bullied Demyx into doing some dirty work for them, they could maybe convince Luxord to join them as well. Roxas could've easily been manipulated into joining them like Sora, and if Roxas joined them it would be most likely that Xion would also have joined up with them. Axel probably could've been persuaded at that point if they got both Roxas and Xion, if nothing else for the sole purpose that he doesn't want to have to kill Roxas or Xion. That would be a little over half of the Organization against the original 6, plus they'd have Sora (and probably Donald and Goofy if Sora's fighting against the Organization).
Another alternative would be to utilize Namine a bit more and manipulate the other Organization members into joining their cause, but I think that'd be a riskier/more time consuming path for Marluxia and Larxene to venture down.
In other words, I think Marluxia and Larxene could've MAYBE taken over the Organization if they had actually spent more time in their planning, but it was still unlikely right from the start.