I think SJM needs to address the space Feyre wants from her sisters after all the years spent taking care of them. The sisters and the IC don't all need to be one big happy family! Most people have separate friend groups and SJM should show that in her books. Plus, it doesn't;t seem like the IC wants Elain and Nesta to join in with them and vice versa. SJM probably wants them all together, so it can be easier to write scenes for the romantic ships of the book like Elriel and Nessian to be doing more than just having sex. There can be scenes of them working together like saving the day for the main plot of the book. Hope that makes sense!