Justbeme, i really feel for you.
My Dh is 39, i'm 25, i know some of how you feel, only the stepchild is his daughter.
I never wanted her to come and live with us, i love her, but i felt it would spoil what DH and i have.
I had our first child 6 months ago, and DH (despite already being a dad) found it very hard to cope with adding a Baby into the equation, but i WILL say this.. the time his Daughter has spent with us has been wonderful, she's been a godsend and made things so much easier having an extra pair of hands to keep baby amused.
I wouldn't say No now if she asked to move in, infact i'd jump at the chance to have her here because my DS adores her.
You and DH need to sit down and talk, you need to make it clear that she's your daughter and if she wants to live with you, she can.