u shudnt have spied. Now that u know, u have to tell ur sister the truth.
You can start by telling her that you love her and dont like to see her die upon the path she's walking upon. If you are practisin, try to get your sister to spend more time with you.
Avoid to humiliate her, or in anyway degrade her. Allah can forgive her, but if you are mean to her, its worse. Imagine, maybe u are her only hope, and if u do something to her that hurts she will distance herself from you. And fall deeper and deeper into the gutter.
Truth is no one wants to stay in darkness, we just need someone kind enuff to pull us out of the dirt and remind us that yes we failed, but we are human and we can change. Allah is the judge. We have to help each other, thas our duty towards one another.