How old is your sister?
In my state, if she is under 18, then yes, this may be a mandatory report situation. If there is any other minor in immediate harms way, that is also a likely mandatory report situation. There also may be other reasons for reporting the abuse that she isn’t sharing with you, due to confidentiality, and is including your sisters comments about the abuse to you as part of a larger report.
But, if those situations don’t apply, it actually seems a bit strange she thinks she has to report this. If therapists had to report every time an adult mentioned abuse, then trauma therapists would be doing nothing else but filing reports all day.
Then again, laws will vary from state to state and country to country.
If it’s a University counseling clinic, I’d suggest calling and speaking to a supervisor. Usually they are pretty tightly supervised, but who knows. A call to the clinic supervisor might help make sure this is all being done correctly.
Also, I agree with what others have said about your right to not speak to anyone investigating the abuse. Totally your right. If law enforcement get a report, it’s all hearsay at this point. It’s what a therapist is saying a client said a 4th party said. It is unlikely to go anywhere unless there is other abuse included in the report (which you may not be aware of) that others verify or you cooperate with the investigation or if for some reason your abuser confesses....
I’d suggest contacting the victim’s advocate for your area if this report does get filed. Get someone involved who is totally looking out for you as you handle whatever might come next. If you are in the US, Rainn.org can connect you to local victim advocacy resources so you really know your rights well and have support in dealing with this matter.