If you already had sex with your sister just to stop her from committing suicide, i don’t think you should second guess this question… or you could call suicide hotline and put her in a mental asylum that would work too
"Truth be told, I love your Imouto-chan and I became your girlfriend only to get close to her, and now the idea of you fucking her before I was able to make a move on her is making me fall in madness"
"Eh... both?"
If this ends up in a threesome, it would be so fricking funny xddddddddd
(I'd choose the girlfriend btw. She's cute and is not a manipulative bxtch. And glasses ftw)