Teddy and Orphans
Remember this tweet? It is from 18 May 2016, i.e. during filming series 4.
The first one shows two little boys, maybe brothers, and between them a teddy. They are holding the teddy like they would hold a third sibling. However, this is not a child but a teddy bear, something that in play may be treated as if it was alive but actually is not. A living teddy bear is something that does not exist, it is not real.
Then, however, in S4 there appears the forgotten sister. Three Holmes siblings, not two. But I cannot stop thinking about this picture - two little boys and between them something that seems to connect them but that is not another human being. Two boys and a teddy instead of two boys and a girl.