Nephew is 5 and a half. Still uses dummy everywhere outside of school. Sister is unapproachable re topic. His speech is already affected.
I know some children really struggle with giving it up and parents at wits end but sister and BIL see no issue. My sister talks to nephew in baby talk, broken sentences etc I think she wants to keep him a baby. I have 2 kids myself, I understand to some extent.
At this point though I think pure parental laziness, they haven't even given him the chance to give it up.
YABU-None of your business let them do what they want, deal with any dental problems later
YANBU-poor parenting, neglecting child's needs