is there some sort of job classification schedule that you could use to support your arguement? i work for the gov't and get paid not too much, but each position is classified based on responsibilities. each of those classifications has a pay range and they can overlap. however, that it could be to your advantage if you list everything you do and can show that you have more responsibility than your sec.
i don't think it's a bad thing to say the sec makes more than you, but i also agree that it can't be your only reasoning as to why you need raise. show the boss what all you have done and see if you can compare it to others in the company who do (or maybe should do) as much as you. this also helps in the arguement "i have nothing to do with her kid" by keeping it more about you than just her. compare yourself to those on a similar level and not your subordinate.
i do like the repitive thing b/c sometimes people just think you will give up.
whatever you do, good luck!