I'd suspect Ayumu threw a bra on to go next door, rather than expecting to sleep in it.
(Though I do wonder what exactly their parents think of the situation/is going on. Maybe Yu's folks were out, or are very heavy sleepers? In SIFAS, the families are close enough that they vacation together. And now I'm getting ficbunnies for a Marmalade Boy -type plot where their parents admit they've been a foursome for years....)
To get back to the story, I like it a lot. I enjoy Kitaku's stories where the couplings get shuffled about, but the ones that are firmly AnaPomu are also top notch. It's also always kind of funny to see Yu's enthusiasm and tendency to throw herself into something headlong directed towards sex.