So just to give a quick update as to how I'm going about this.
I am waiting until Thursday for the second pregnancy test before moving forward.
Until then, my little sister has finals this week so I've been studying with her and using it to get her mind off things. Also making a lot more jokes and being lighthearted just so she isn't overly stressed out about this, but not too lighthearted to the point where it's weird and it seems I'm trying too hard.
Assuming the test is negative, on Thursday I'm going to reach out to the boy and schedule a meet up place probably at a Starbucks with a few isolated seats where we can talk. I'm not taking "no" for an answer for whether or not we're having this talk. During this time, before I say anything I'm going to let him air out his side of the story (in the case that he sheds new light on the story that would indicate my sister is unfairly shifting blame), and I will slightly tailor my following response based on what he says.
Basically I will tell him that what he has conducted is statutory rape and that if he ever ever texts my little sister, meets with her, or reaches out in any way I will without hesitation call the police and tell them everything that's happened. In the unlikely case, I somehow ever find out he's also doing this peer pressuring to another girl, I will also call the police. I don't want him doing this shit to anyone ever again. Based on what my sister has told me, dude is scared enough about this whole thing that he should listen to what I say. Just a slimy senior trying to lose his V-card before college by pressuring a freshman girl. If he tries to start shit and fight, I'll have a buddy of mine nearby to help me restrain him and calm him down. I'm not looking to fight anyone.
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Now to answer some frequently asked questions.
When and why did she tell me this? She told me this morning after I noted she was failing in two of her classes, and she quietly mentioned she was stressed about things I didn't know. After prodding her a bit, she opened up. I'm a no-nonsense person with a relatively calm mind when it comes to stressful situations, so she opened up to me so fully because to a certain extent she trusted me when I said I would help her with no judgements.
Why not just beat him up? The only pro I can see from this is maybe intimidating him and getting my frustrations out. The cons include hurting someone my sister doesn't want me to hurt, making this possibly a public thing as word gets out I beat him up because he fucked my sister and she gets more embarassed, I get my medical school acceptance revoked due to the altercation, my parents find out and are eternally ashamed of both me and my sister, this ends up going to court and authorities get involved, etc. Like someone else said, this is not a Hollywood movie where your actions don't have real life consequences.
Talk to her and make her feel OK. I have, I had a long emotional talk with her when she told me. I let her know that she shouldn't be ashamed of sex as it is a normal thing that many people do in high school. The only issue is the context of that sex, and if he pressured her into it. She doesn't owe anyone her body. She has agency. She can get an abortion if she so chooses (but on her own accord she adamantly refuses). I also understand the fears she has about this getting out to parents, school faculty, etc. so I let her know I am completely in tune with what she wants, and to give her some semblance of control I am letting her know everything I am doing before I do it. That doesn't necessarily mean I'm waiting for her permission, but only that she has time to give her input before I do something that could jeopardize her. I held and hugged her while she cried this morning. She knows I have her back.