It is a little tricky to file a police report without any evidence because it could be taken as a false accusation. I would go to the police station in person and ask if you're able to file a report but let them know you're wanting to file one just to have it on record in case any other young girls report them in the future. Don't use your family members name or even let them know it's a family member until you know if they're going to let you file one. They may not since there is no evidence but since you asked first before actually reporting it you're able to know if it's possible without actually accusing them. They will ask you what happened so they can document everything. But just make sure they know you're not pressing charges (I don't think they'll write articles or anything because unfortunately these types of things are reported all the time, and especially with no evidence, it's not a riveting story that any news establishments would want to cover.)

I would also keep a close eye on any younger cousins or siblings you have to make sure they don't endure the same trauma. Best of luck!