An uneasy alliance between the thirsty gay girl and the imouto she fears as competition begins...
Also... Bizarre to say the least... That the imouto is willing to sell out her sister that way... I mean, at least she's "selling" it to a lifelong childhood friend that thirsts after her and wished to do gay girl things with her and not, y'know, the male character of a degenerate NTR doujin or something really sleazy, but still... Imouto better hope onee-chan don't find out.