Yeah I am not sure this is GL.
The sister seems to be yandere towards her own brother. She seems to be training the GF and have sex by proxy with her own brother with her help. Not sure if she just uses her for said proxy, or genuinely wants to help them get frisky and close together with her having "blackmail" on the GF to continue the sex by proxy.
The dude underperforming cause he not experienced, and girl kinda preferring the sister abilities of making her climax and stuff, and the sister seemingly not seeing what she does as a cheating/fucked up thing to do, leads me to believe there will be some drama. Like NTR type of drama, but not the "huge D stealing girl from small D fully planned NTR" but "girl was teaching other girl how to be frisky in bed, and made her a lesbo pinning after her by mistake NTR".
Bro will find out at some point and what he will find will devastate him, the GF and sister and his relationship will either fully go into tethers, like bro will break up with the girl and tell the sister he hates her and doesn't want to see her, and the girls will be devastated or at least sister will be, cause GF could been at that point fully "brainwashed" and in love with the sister or something. Alternatively there will be an "try to fix everything" ark of them trying to fix stuff and maybe ending in a 3P relationship of sorts.
Dude could also not find out about the cheating/blackmail/things sister does. Just that the GF and the training sister puts her in, would slowly destroy her as a GF candidate and ruin hers and bros relationship. Which would make the bro hurt and it would be purely the sister fault - so like still the "NTR by mistake" drama, just with less emphasis on the guy being destroyed by knowing what was happening behind his back, and more of him just being sad cause his relationship and sex life dies out.
But then knowing my luck - it will be something completely different, cause thus far like all my theories die cause the author was retarded and gone with something so stupid and out of his sorry ass, that I wouldn't believe someone will stoop so low, or author was a "genius" and came with a plot twist I wasn't able to theory-craft myself
