Pretty straightforward. The middle one's (I can't seem to find her name) brother married Tsubomi, the eldest of the girls here, and later died after presumably having the younger girl there. Middle girl fell in love with Tsubomi but kept it a secret. Somehow the niece figured that out and came up with the "I like girls" thing in order to make the aunt open up about it. No telling whether she really is attracted to girls or not, that's not really the important part.
I think the title is a reference to the flowers by the late brother's memorial photo, since they'd continually be kept fresh. In the same way, the aunt's feelings towards Tsubomi aren't changing either, they're just being kept there. And Tsubomi, at least, is content with that.
