Pop Goes the Weasel (Part 3) by Amethyst

or perhaps when she's due for release?

Yup, that would make sense,

wouldn't Scamper's therapist be obligated to make sure that information gets passed along when she tells them?

Depends on what is said.

Yup, as Domo said, it depends on what was said and if specific people are named. But if they informed Lily or her guardian, they would have to ensure not to reveal their source, nor anything about her beyond that information, including her relationship to Lily.

Lucia told Scamper that Arthur Pond had raped Lucia and was the father of Scamper. Lucia also told Scamper that she would be going after Lily Pond as well. Now, in some jurisdictions, there might be prohibitions against forcing Scamper to testify against Lucia, but I don't believe that Lucia has any grounds for complaining about Scamper revealing everything to others. (Doctors, lawyers, and priests might have some limitations on what they can say about people they deal with in certain situations, but I don't see where Lucia would have such a relationship with the other characters.) Authorities could use DNA testing to verify that Lily Pond and Scamper are half-sisters. I also don't see any legal restraint on Mrs. Pond providing information. I therefore don't see any expectation or legal requirement of confidentiality regarding presenting information about Lucia to the authorities.

I would expect Scamper to be assigned a therapist at Whateley, and that would be a good source for considering possible risks of Scamper talking to Lily.

There shouldn't be many barriers to proving that Lily and Carmen are sisters, and Lily should be warned of the threat toward her. Given that the Nuevo Rangers have access to the recording on which the threats were made and have some contact with the All Stars, it is possible that they will pass on the warning that way and possibly reach out for both Lily and Carmen's sakes.

And yes, Scamper will be seeing a therapist at Whateley, She should be a good sounding board for Carmen in addition to helping her with her abuse trauma.