So it seems they are cousins, not brother and sister. I've changed the tags accordingly.
The first one to pee loses, alright.Niichan, what's this~!?Ah!
Come on!You, go take your bath...I wanna do this too!Huh?
You don't know what sex is?So it's called sex!?
This!Seems interesting!Okay...This, it feels kinda weird...