I feel like Miyako has had a crush on "Onee-san" for a while, and this is her finally realizing it. Her saying it out loud there comes across less like a confession, and more like a thought voiced out loud. She doesn't seem like the type to angst much over it, so I'm betting the next volume is going to be her going on the offensive to win Suga's affections (with adorable and hilarious results).
Meanwhile, I don't think Asako is at that point yet. She clearly really cares about Miyako, but it's more of a protective and affection kind of caring, rather than romantic. It's likely that she's still burned from her last relationship, as well as all the logical reasons why Miyako might seem "off-limits." (she's younger and less mature, she's in a position of authority as her landlady, she's a former idol so there's a gap there, and - the big one - Asako probably assumes she's straight) I think it will take a little while before Asako starts to fall for Miyako that way, and likely even longer before she'll allow herself to admit it.