The pattern of acquisition for relative clauses (the “accessibility hierarchy” for relative clause in English):
Subject (‘The girl who was sick went home’)
Direct object (‘The story that/which I read was long’)
Indirect object (‘The man who[m] I gave the present to was absent’)
Object of preposition (‘I found the book that John was talking about’)
Possessive (‘I know the woman whose father is visiting’)
Object of comparison (‘The person that Susan is taller than is Mary’)