going against the grain and say ESH...while I do think you were right to mention that the books SIL is reading contain unhealthy romantic/explicit stuff. She is exactly at the age where romanticizing unhealthy romantic/sexual behaviors can become dangerous, as you don't have the knowledge and life experience to know much better. But going behind her back to have them confiscated is just teaching her she should be secretly reading that stuff on her phone.

What she needed was a conversion. Why did she read those books? Does she like those romances? Does she think they're appropriate? Does she understand consent and healthy boundaries? Right now she's probably thinking her family thinks she's too immature to come into contact with explicit material, that in your eyes she's just a naive little girl who can't handle the realities of life and should double down and go read up on and find out more about this type of explicit, unhealthy stuff.