Why not just make up words in english? Ok, I know that sounds silly, but I'm thinking more, use terms that may not be the standard 'king, queen,', but still get the message across and aren't strange words that are - and I think this is the big hang up- similar looking. Plus, I think working on a similar set-up as words that do mean king queen prince princess (i.e. tsar, tsarina, tsarevitch, etc, which your titles reminded me of) may have the exact effect you were hoping to avoid.
I know that's why, personally, such things tend to be a stumbling block for me. I start feeling like I have to memorize vocabulary and concept lists. That's fine in a foreign language literature class, but it's not fun.
That is of course the great thing about english.. the language generally has three or four ways to say the same thing, sometimes more, because of all the different languages that lent it vocabulary. (e.g. kingly/regal/royal). Plus, if there is any word-associations that are remotely similar to your set up, maybe use them.
I know, using an example from one of JB's books is overdone, but look at the codex alera with 'first lord'. He could have used king or some derivation of Caesar. But his chosen term was easy to remember, understand, *and* it implied something of the political set up. And the great thing about history, is that there has been such a variety of different systems, all with their own unique things, that perhaps there are bits and pieces of them you can steal from.