DS age 3 is absolutely obsessed with role playing families "you be mummy dinosaur, I be baby dinosaur" or you be "grandad monster and I be baby monster" and on and on. Every situation requires him allocating family roles for everyone. He is almost always the baby although he hates being referred to as a baby any other time. His older sister (now 7) never really went through this stage but she doesn't do pretend play much at all and still struggles to do imaginative games.
The difference is that their Dad and I separated 18 months ago and he has since had a new baby with his new girlfriend. Should I be worried that DS is struggling with the two families thing or with understanding having a new baby sibling whom he hardly sees? He's otherwise well adjusted, doing fine at nursery, speech is good, probably slightly above average in his development.