By the way,
Shadesishere.
Words like 'Chuu' & 'Rero', if I remember correctly, are meant to be words for sound effects of Kiss and Lick.
Though, I don't really know if it's better to translate that and put something like **; considering that it's in the dialogue as if the sister actually says it.
I have seen it actually being said in some animes.
(Rero in that popular scene from Jojo's Bizarre Adventure, and a character in D.Gray-Man who uses that as a suffix or something after every word/sentence.)
Can't read japanese, so this isn't certain, but these could actually be one of the sfx that pops up in sex scenes.
(Also, could you capitalize the 'no' in the Yes or no when peeping on the sister when she goes to the toilet? Just a little nitpick but it bothers me, so I just really wanna say it.)