Out of curiosity, is the grandmother your stepdaughter's paternal or maternal grandmother. I'm guessing she is the maternal grandmother--and I'm wondering if she resents you being the mother figure in the child's life since she's lost her own daughter, and in a way, is somehow trying to re-live her motherhood through her granddaughter because that's all she has left of her own daughter.
If she's your stepdaughter's paternal grandmother--she sounds like a bit of a control freak, not to mention insensitive about telling her granddaughter that you're not her "real" mom. I think what you need to step back and realize in all this, however, is that your stepdaughter's behavior is a result of a grandmother spoiling her rotten and a father who approves of it, but this is not your stepdaughter's fault. She's just being a kid.
I think what you need to do is talk to your husband about the grandmother, especially if she is his mother. He needs to understand why you're concerned, but at the same time, you have to present your case without it seeming like you're attacking his mother or daughter's behavior. Perhaps some family counseling would help.
If the grandmother is the maternal grandmother, maybe you could try talking to her personally and reach some kind of compromise about how much involvement and authority she has, giving extra sensitivity to the fact that you are trying to reason with a grieving mother who is probably trying to cover her grief with defensiveness against you and "reliving" her daughter's life through her granddaughter.
Maybe even the whole family might consider doing counseling together so you all can learn to communicate what everyone's expectations are with as little conflict as possible.
You say you're the only mother the child has known for over 5 years, and then you say the child is a "real brat," expects everyone to drop what they are doing and attend to what she wants, interrupts, won't heed you telling her to stop when two adults are talking, rolls her eyes, infuriates you, and you have had enough of this crap. It sounds like you feel judged by the family and unsupported by your husband, and it sounds like the child is a flashpoint for power struggles with your mother-in-law. It sounds like a lot of this is about whether or not she gives you the power of your position as your husband's wife, more than about the child. But whether or not that's your intent, the long list of complaints also sounds like you don't love the child.
I guess in your shoes, I would decide if I do love the child, and if so, then function the way a person does when they love a child unconditionally. If my son interrupts two adults who are talking, we do talk to him, at least for a moment, so his emotional need is met. It doesn't cost much in the conversation, and it builds him up for a future lifetime of feeling like people think he is an OK person. There is just a way an adult behaves toward a kid if he or she loves them unconditionally. Stepparents are usually writing on this site about how they want their spouse to learn to discipline the child so the child behaves with respect. Nobody ever writes in saying their stepchild is wonderful and they are wondering how to love them more.
If someone took my son to all the movies, or threw my son a big party on his birthday, wouldn't that be nice. If you feel grandma is somehow doing things like this as a challenge to your position as mother, you could ask her to tone it down. (Or you could talk to a counselor to see if this reaction is being overly sensitive. A lot of parents would be delighted if grandma eased their load on stuff like going to the movies. And, lucky you that she watches the child in the summer! Do you know how much money that saves?) Of course, grandparents do dote on their grandkids and some grandparents (my father-in-law was one) can't be stopped from buying a lot of gifts. Try not to take it as putting you down, grandma is going to keep doing it because she loves her grandchild. She might also think you don't love the kid very much, and be trying to make her grandchild feel better about the situation when she says you're not the child's real mom. (After all, this way the child can rationalize that her real mom loved her unconditionally even if you don't.)
If you decide this child is always going to be an issue in your life and your marriage, you might consider leaving. You can't ask a dad who loves his child to turn away from his child for you, it is just a bond that can't be torn apart and should not. You're a grown-up, you could move on. She's a child, she is forming her opinion of herself and the world. She right now is obviously sensing that you disapprove of her, and that's a tough thing to live with at 7. Her becoming a hellion who thinks the world owes them a living is probably as likely to happen from the resentment and disapproval in her life now, as it is from any lack of discipline. She's not going to grow up feeling secure in her world if she is living with a disapproving adult who comes down on her hard for her natural reactions to the emotional dynamics that are happening around her.
Many small children xxxx their thumb and mumble that first few words said it all ...I doubt it will ever work... find someone without any children..
michie, your posts make me so sad for this girl. Give in to her grandmother, who loves her.
I'm just sad reading all you have to say.