Have her tell her school nurse - and your parents or guardians.
School nurses are mandatory reporters, and it should get more attention if she reports it through school. Your parents/guardians can also demand the police do more. Sometimes kids making reports aren't taken seriously, which is wrong - but then, sometimes there is more drama behind the story and they've heard enough to think this might might another case of that.
Have her write down everything that happened, and if there were any other witness, have them write things down too. Unfortunately, the police are correct in that they need evidence, and just one person saying it happened often isn't enough - even if it did.
One of the most important things is not only to avoid this guy, but also to get your sister to talk about this to someone. Even a short incident like that can shape the way someone feels about other people, so minimizing the damage to her is as important as stopping him.
Here's where she could start:
https://ohl.rainn.org/online/
"Justice" isn't something you go and get - and often, there is none. If this guy keeps on doing inappropriate things, someone will get him - it shouldn't be you. Going after him, or having anyone else do it, will simply rob your family of their good reputation- and put you in jail. Don't.
Get your sister help, and tell her to speak to her school nurse when she can.