The more chapters get released, the more convinced I am, this will end up in a tragedy/sad ending. No clue what and how things will happen, but the end result be - the plot device MC will end up without a girlfriend, and angry at his own sister enough to end any contact with her. Could end up being a cuck that has ED issues till he sees his girl fucked by others - but that would be a cope out solution.
The sister would end up broken, cause all she wanted is to help her bro and his girl get together for good, cause they were shy dorks, and she thought that if she helps the girl get a bit more frisky if you will it will help them both - just that she failed to realise she could as well create a unsatisfiable monster that won't ever be fullfiled while sexing the dork "virgin energy" MC.
And the most fucked up will be the innocent girl. Girl was in love, had a boyfriend, had a decent relationship with both him and his sister, then the sister borderline yuri raped her, started training her, thought her things that she now wont be able to live without, the relationship will break with MC and the sister, leaving her lonely lusting after pleasure - probably ending up a toilet prostitute like the cute small breasted girl from this new ark.
Sure it could also end with the trio all happy in a perverted siblings with a 3rd wheel relationship, or we could find out sister was from the get go after the girl, to rob her own brother of her, dude cucked behind the scenes and sister and her lover girl in a yuri relationship. But at this point it would be the lamest ending.