Ok it completely is awful that the woman who answered your call was so incredibly unhelpful. BUT I have 4 bits of advice:
1) Is there anything you can do to get an accurate address? Have you done any basic internet searches, or looked at the online yellow pages - anything to try to track down an address?
2) Call again. I swear, it is amazing how 2 different intake people sometimes respond to the same information. You might give the exact same info and another worker will take the call. I think it'll help a lot to have the address though.
3) Think about how you described it on the first call. If you didn't do this already, make sure you lead off with the fact that the child is only __ yrs old (is it 3?), and is making and doing sexual things that are simply inappropriate for a child that age. That's how you feel about what you know, that's how many of us who heard your description feel, so don't worry that you're not a professional expert on child development or something. It's ok to say clearly and definitively what YOU think. It's up to CPS to look into it and figure out if they agree or not.
4) If you call back and get a 2nd similarly unhelpful person, don't hang up without asking them - calmly - what else you should do since them refusing to take this report means a child who is acting out sexually in totally inappropriate ways and is living with a pedophile is still in danger. Ask them what else you should do since they're lack of response is leaving him in that situation. SOMETIMES - not always, but sometimes - that makes them listen to you a 2nd time.
You are brave and wonderful for trying. I am hoping you just got a sucky intake person, but I can also understand a bit how they may feel like it's not enough to go on, especially without an address. But if you can track down an address and call again, and focus on the general fact that it's a 3 yr old acting inappropriately and showing signs of molestation, then give all the details you know, then at least you've given it the best shot you can with CPS.
Best of luck, still totally and completely rooting for you and your nephew and the whole family to recover from all their pains and issues. Take care!