Is my sister falsely accusing ual abuse of her son illegal?

Honestly, I'm surprised your sister hasn't lost custody of her son. The ex-boyfriend sounds great, and I'm surprised that she would break up with him.But yes, false allegations of ual abuse are illegal. I agree that you should talk to the ex-boyfriend about getting lawyer. The problem is, if they weren't married, I doubt he'd get custody. If your nephew wants to live with the man he considers his father, I'm sure they'd at least consider it. I'd suggest you keep an eye on your nephew, though. It's possible that your sister might actually convince him that the abuse happened.