Dear A.,
I'm 45 as well! And oh, how I know about the step-mother thing.
I married a man 10 years older than me. My daughter turned 5 years old, 10 days after our wedding and he had two teen-aged sons from his previous marriage. I've never known a kid I couldn't get along with and I had no idea what I was in for. It's complicated, but the oldest boy was raised by and lived with his maternal grandmother. He was 15 and an angel. Very polite and respectful, loved my daughter and played, colored, read with her. The 13 year old was a completely different story. He lived with his mother who was very antagonistic even though they had been divorced many years before I even met my husband. At first, I thought he was just having a difficult time adjusting to everything being new and having a new step-sister, etc. and I certainly had no interest in being his "mother", he already had one. As the years progressed, his levels of utter disrespect and selfishness only got worse. Every time he came to stay with us, it was hell. He made it clear to me that it was not my house but his FATHER'S house and he would do as he pleased. My husband completely allowed it and blamed his mother for never teaching him any manners. If he took a shower, he left his dirty clothes all over the bathroom. If I asked him to put them in the hamper, my husband would march in and do it. The one and only time my husband EVER stuck up for me was on a day that the boys were leaving and I asked them for their laundry as I didn't want to send it home dirty. Little Mister informed me that HIS clothes got washed by hand and hung dry or I could forget it. My husband, assuming I was picking on his son, came running into the middle of it to rescue the kid from me. When he heard from the kid's mouth that he expected me to bend over the bathtub to wash his clothes by hand, his dad informed him that the washing machine was good enough for his dress pants and shirts, so it was good enough for his jeans and T-shirts and smelly socks. We got calls constantly from the school that the kid had an attitude problem. He cut 43 out of 45 days at school. So, my husband bought him a truck and paid for all the insurance and gave him plenty of gas and spending money. He lied constantly. I caught him going through our desk and financial stuff and on the phone reporting it to his mother! All my husband had to say was, "He wants her to know he's proud of me." Every time I tried to say he should come and live with us, or he can pick up his own dirty laundry, or he needs someone to tell him his rudeness will not be tolerated, it caused a HUGE fight and I was accused of hating the kid. And being jealous. Now, mind you, both the other kids had rules and bed times and chores. But not the little prince who, at dinner one night, in front of all of us, informed his father....."You don't tell me "NO". You will NEVER tell me "NO", do you understand? You will do what I want and if you don't, I will never come to see you again." Everyone was in complete shock. Except my husband, who sat there eating his dinner and pretending like he never even heard a single word. Things went from really bad to intolerable when I got pregnant. That kid's demands went through the roof. And my husband catered to every single one, no matter how insane. My marriage only lasted a year after the baby was born.
That said, and sorry for talking about me, but I do want to try to give some advice.
First of all, girls are different than boys. It may be that your husband feels awkward talking to his daughter about hygiene, her self esteem, her feelings, etc. So, in a non-derrogatory way, you have to try to talk to your husband about your concerns so that you can be on the same page as far as parenting. My own biological daughter went through a phase where she was completely unruly and I would swear she was not the same kid that came out of me. Like somebody body-snatched her or something. It doesn't have to mean there is something "wrong" with her, but if you lose communication, it's hard to get it back and the communication needs to start with you and your husband. I mean, is he thinking that she will be 18 soon and moving out on her own so you can just ride it out til then? You have to try to get to the honest bottom of HIS take on things. What are HIS hopes for her or HIS feelings about her slumping grades and perhaps not caring about herself or her appearance. Start there. With you and your husband. Tell him that you do not think it's unfair for her to pick up after herself. You don't expect her to clean the entire house, but if she leaves a cup on the coffee table, it won't hurt her, when she's done with it, to put it in the sink or the dishwasher. If you want to talk rationally and non-judgementally and he turns it around on you, you know you have a problem that goes beyond the daughter.
If your husband isn't on board, then I would take a different approach with the girl. Maybe she feels you are trying too hard to "help and guide". That tactic hasn't worked, so try saying to her that you really do care and she can come to you to talk about anything. Then, step back a little. If she is stubborn, like my own daughter, you can't try to outwardly "turn" her into anything. If that's what she thinks you're doing, she will blow you off. My daughter who will be 22 in October now says, "You were right. I should have listened." But at the time, listening to me was the equivelant of selling her very soul to the devil or something. She is so incredibly intelligent and started failing classes in high school. I guess she thought she could get by on her charm. Nothing I said or did made any difference. Then she tried out for cheerleading one day and was asked to leave, in front of everyone, because of her grades. THAT got through to her. And she started working her butt off. You can't have everything without doing something. That's just a rule of life. Without seeming like you are hovering or intruding, try finding out what your daughter likes or what interests her. Even if you don't like it yourself. Whether it's music or whatever. Maybe plan a day to go to lunch and hair cuts and manicures. Even if she wants black nail polish, let her have it. The time is more important. My daughter dyed her blond hair almost black, kind of spikey and punky looking and I was so mad. But you know what? I have sent pictures of her to family and friends and they all say, "Oh, my God. She is SO cute!" And she is. It's her style....not mine. She's an individual. So, try to find something that you and your daughter can do in common and have fun with. Anything to open the lines of communication. So far, it doesn't sound like she's done anything SUPER terrible. You love her. She's not a problem. She's not THE problem. Let her know that and get to know her on her terms. Be patient. She doesn't sound nearly as bad as she could be. Trust me.