I completed don't get the plot.
Ri-chan had a crush on her brother’s fiancée, but that clearly didn’t work out. It’s most likely that she never told the woman her feelings and kept it her own secret, as evidenced by the old secret crumpled picture of the woman that she had hidden in her old yearbook and the somewhat wistful attitude she treated her with. Mika was able to piece these things together and either just realized her own feelings for Ri-chan or already knew. Either way, she decides to start acting on them instead of waiting and keeping it to herself, since that could lead to “losing” her, kinda like how Ri-chan lost the fiancée to her brother. Mika gets Ri-chan to teach her the organ in order to get closer to her. In a few years they will probably be happily together and fucking like rabbits.
And yeah that’s pretty much it.