It's never really been gone into either way, the main basis of the theory seems to be the fact that we're never told anything about Arika's father either way. Though I would *think* that it would be enough of a scandal that we'd hear about it if the king was shagging both the queen *and* Rena. And he does have a proper wife (Sifr Blan, new character in Otome Sifr) so Rena can't be the queen.
I'm thinking we'll be at least introduced to the person who goes on to disqualify Rena from being an Otome in Sifr.
...I'm also hoping we get to see Arika's grandmother, too.