3 year old adjustment

My daughter is 3 years old, and she seems to be having a very difficult time adjusting to my girlfriend and her daughter moving in. Since she was 1 1/2 I have had full custody with her visiting her mother everyother weekend. I work nights so I have had to place more of a parenting role on her grandmother then I would have liked. My girlfriend moved in 2 months ago and my daughter's attitude has gone down hill since. My girlfriend goes out of her way to do extra special things for her ie.. painting her nails, doing her hair, dress-up. Grandma's role has diminished majorly with her also going on vacation for a month also. I think my daughter is having trouble recognizing my girlfriend as an authoity figure... I know they love eachother because they have know eachother from my daughters birth. My daughters bad behavior includes sticking her tongue out at my girlfriend when she tells her what to do, grunting and whining when told to to something, as well as being mean and sometimes physical towards my girlfriends 2 year old daughter. The three of them went out to eat at a resturant while I was at work adn my daughter was terrible. She said she didn't have to listen, that she wasn't going to eat her food, and throughing a screaming tantrum when given a slight swat on the butt.  This type of behavior has carried over to daycare, which used to be where her best behaior was, and I can tell she is pushing them close to their limit.  I need all of the advice that you can give!!!
                                    Thank you,
                                                Dad

[Kevin Kennedy, Ph.D.]

Try not to underestimate the degree to which your daughter's life has been turned upside down. Your girlfriend and her daughter have moved in, and your girlfriend has assumed jurisdiction over your daughter. In addition, your daughter faces the changed role of her grandmother. It's not surprising that she's having some trouble adjusting. Of course you have to insist that she follow your girlfriend's directions. This insistence should be accomplished in a firm, decisive and calm way. Swatting her is not a good idea - employ time out as an alternative. The fact that your daughter has known your girlfriend does not compensate for the reality that your girlfriend's role has changed. She is now in a parenting role.