I like to imagine Yuna and Yume playing with their water guns in the garden shooting each in lewd areas with their clothes becoming wet and see through.
Later they try to help each other out of their wet clothes only to start feeling a little naughty at the sight of each other's wet bodies. So they begin to play new more intimate "game" only to notice too late that Onii-chan has been watching the whole time and he wants to play too.