I understand that this manga is about the Oomuro sisters (the clue is in the title), but I really appreciate the rare times we see Nadeshiko's friends interact with each other without her present. They go a long way towards fleshing out their characters, and can be greatly effective in adding intrigue to the "secret girlfriend" plot thread.
Also, I don't think this chapter necessarily confirms that Megumi isn't the secret girlfriend. Some of her expressions throughout the conversation (and I've said this before on these forums, but Namori does not get enough credit for her how well she does expressions) could very well be interpreted as guilt.