Sure keep pushing him you will only have yourselves to blame when he finally snaps and nails one or both of you.
If this were an actual ero manga story I could see the Onii-chan character eventually caving and doing it with his sister's friend. His sister finding out and getting upset and jealous deciding to try and bed him herself.
He will resist a little because 'siblings' but ultimately gives in, maybe ending in threesome with her friend joining in.
There is a two part story by Shirane Taito that is a little like the thought I had above called PapaLove as you can guess from the title it is father and daughter rather than siblings.
Also it seems Airandou has finally released a collection of ever thirsty shark loving imouto doujin.
There is a two part story by Shirane Taito that is a little like the thought I had above called PapaLove as you can guess from the title it is father and daughter rather than siblings.