I'm going to be in the minority here and agree with the advisor. Considering the situation, I don't think the prince should concentrate on being housemates or seeing her wife as child, regardless of the fact that is who she is. He should be seeing her as that, his wife. Now, I'm not saying he should consummate the marriage at such an early age, but seeing the relationship as platonic, I don't think that's good either. A compromise is best: he should not engage in anything with her until she is of age but should recognize her as his partner rather than some kind of family member.
And I'm well aware everyone here does not like that idea. I see why you guys don't like it, but I'm considering the setting and the political situation. I think it's for the best if the Prince see's the Princess as who she is in this situation.
And this is an age gap romance after all.