This is called spousification, and it is all about the blurring of the boundaries between adult and child roles in a family. You aren't the one with a problem here, and don't let your DH make you feel that you are.
Sometimes it results in the daughter (or son) carrying too much adult responsibility, as when the daughter steps in to take over the traditional female housekeeping roles, or being too responsible for the father's emotional support.
Sometimes, the daughter only takes on certain more 'attractive' aspects of the maternal role - all the privileges and power of the adult role with none of the responsibility.
My DSD fit into the latter category. I know exactly where you are coming from. I spent more than two years being the housekeeper/nanny for other DSC before things really started to turn around. It is much, much better now.
I should say that I never felt that my DH loved his DD more than me, or felt that it was a competition - for one thing, it didn't look like 'adult love' - it was more that I found the whole thing incredibly irritating and it got in the way of our growing relationship. I wanted to be treated with respect, and frankly, it's not respecting your date/partner/wife to ignore them, sideline them, or refuse them privacy, or expect them to not act as head of household in their own home. I put my foot down, hard.
I recommend this book a lot, but I think it is particularly useful on the subject of roles and boundaries around the adult relationship. It is called Stepcoupling, by Susan Wisdom.