[Rafal]

Originally Posted by Green²

"No, it's just that Mukuru is still just too young."
"Young, you say?:"

Spoiler for NSFW? Ep.5:

(Check back with xris before using.)

If Mukkuru is female I doubt she'd be behaving like that, unless she's confused about her own gender.

But one perhaps better argument for Mukkuru's gender being female lies in the last episode 8, when

Spoiler for episode 8:

Mukkuru is lying in the grass with the girls, like a mother feeding her own cubs.

As for the twins,

Spoiler for game:

they're 100% male in the game, although Hakuoro is quite convinced they're female until he's told otherwise.