NGL would be funny in an NTRy way, if the training that sister does would actually fuck up the girlie brain and make her a lesbo for the sister, ala the typical NTR porn where the girl "finds out how it feels with 10 incher rearranging her insides, and stops being able to feel any pleasure with the smoll D of her original love interest".
Like the sister trains and trains her, while the girlie isn't able to go fully with the dude - like it seems it happened multiple times thus far - and somehow the girlie developed the "it needs to be sis or no pleasure" thingy.
Bonus points if it fucks up the sister too - when she finds out her training didn't do what she wanted aka instead of preparing a human cum bucket for her brother, she made a lesbo pinning after herself.
Thou with how the sis seems to be able to self-play often imagining the girlie and what they did in the training - it could be simply that she actually wants to torment her bro, by "stealing" the girlie.