Boukansha no Koi

"I want to be by my sister's side until the last moment..."
In the fall of her twenties, Rachel married her childhood friend Noah, who had been her longtime crush.
She knows that the person Noah really loves is Alicia, his older sister and Rachel's best friend...
Noah and Rachel live a married life full of deceit while Alicia has only a short time left to live.
Rachel feels as much guilt and heartache as she does happiness, but who is the real 'bystander' in this love?
https://www.cmoa.jp/title/269197/