4 year olds not potty trained

My nephews are twins and they turned 4 in March. My sister hasn’t potty trained them and says she is “waiting for them to show that they’re ready.” Is this normal these days? The boys are your typical energetic, fun, spunky 4yo boys, but they are still in diapers/pull-ups full time. They also go to a preschool that doesn’t require kids to be potty trained.