Estelle/Joshua are basically childhood friends turned romantic partners that happened to live under the same roof more than anything
They also met at age 11 & didn't exactly grow up together from a super young age. Joshua developed feelings for Estelle shortly after they met too
Estelle initially *tried* to play off as a “sister”, but a major part of the story with them is her finally coming to terms that she also has feelings for Joshua & the idea of them being siblings was nothing more than an illusion. The game literally spells it out for the audience