Will a judge give me full custody if my ex is abusing his stepdaughter?

My ex and I have two sons, age 8 and 10, and we share custody of them. My ex has remarried and now has a stepdaughter. My oldest son says he’s seen his dad do things with the daughter that don’t seem right. What he’s describing to me sounds like sexual abuse. My ex has never hurt my two boys, but I don’t want them around this behavior. My oldest doesn’t want to go over there anymore since he told me. I want to report what my son told me to child protective services but I don’t want my ex to find out that my son shared this information. Can I keep my name and my son’s name confidential if I report it and can I get custody of my boys even if their dad isn’t abusing them directly?