Yeah, I really hate to say it (and I hate that it's almost certainly true), but going to the police will likely be more trouble than it's worth, OP.
When it comes to rape, unless the narrative is that she fought him off vigorously, while shouting "no" and sustained numerous visible injuries in the process, it's unlikely the matter will be prosecuted and even more unlikely there will be a conviction.
Statutory rape is a much clearer charge, but even then - Word will get out that your sister had sex with this guy and, for some, the narrative will be that your sister willingly had a sexual relationship with him and then ruined the poor kids life.
Going to the police is something that the two of you need to think carefully about and something that, absolutely under no circumstances, should you pursue without her consent, because if you involve the police she will be the one dealing with all of the extraordinary burden that entails.
Pretty much this. Going to the cops should be a last resort in this case. Honestly the guy didn't even seemingly threaten her so proving it was rape is going to be insanely difficult.