This sis-con dude plot is kinda genius on itself. Now the "main character" dude who said he is in love with "girl like main girl" has a test in front of him. Either he loves the "girl that looks like MG" and thus if sis-con brother will stop doing his cross-dress antics he will start getting interested in MG, proving himself as a lame dude going for looks only. Or he loves the "girl looking like MG personality" and thus will need to cope with finding out that said girl is actually a sis-con cross-dressing bro - and clash his crush with gay reality.
If protag-kun goes the "personality" route - we will have a funny yaoi side plot, where the "NPC-character helping the girls" will start collecting his actual females like pokemons for his harem if they introduce more girls, or just will have 2 girls typical triangle drama-romance.
Alternatively NPC dude will help everyone including cross-dress-kun and everyone like in a lot of stories will develop feelings for him, and dude will have a harem, just quite varied - cause included in it would be a protag-kun going gay for him, already easily cross-dressing twink who could go gay for him and pretend to be girl most of the time perfect, and the girls already in the plot - just would need 2 more as weird as possible characters added for a proper sextet harem

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So many possibilities.