Help her. Seriously.
You’re neglecting her and if school start looking into this for the safeguarding issue it is, there may well be repercussions for you & DP as well as people questioning your capabilities with a new baby. (My work is in this area.)
If you help DSD, stop being so bitter. Stop actually neglecting the child in your care and love her a bit. You’ll all feel so much better. She’ll be older in the blink of an eye. She’ll probably also be your best babysitter if you play your cards right.
If she’s with you F/T please step up. This odour is a sign of puberty. She’s going to start her periods soon, she needs help. I remember a girl at school who lived with her dad and dads GF coming to my house for tea at 13 and no one had told her about periods or anything and she had nothing ready for them.
Mum made dad stop at Tesco on his way home to buy a washbag, bubble bath, deodorant, paracetamol, sanpro & a 5 pack of dairy milk for her and made sure she had our number, and would send me into school with a pack of bodyform and a freddo for her every month after she started until we left school (she used to leave these on our beds when we were at school too, a day or two before we were due on, she said she could tell it was coming by our moods!) . Mum and I saw her just before lockdown and she hugged my mum and introduced her to her DH and DC as “my fairy god mother”.
It will cost you about a fiver to get her some deoderant, sanpro and other bits and you only need to prompt her to bring washing down, and pop it in the machine. That’ll take you all of ten minutes.
I get your husband should be doing it. I really do. But you’re an adult and she’s a kid. And she is a kid in your care. When you moved in you assumed responsibility and turning a blind eye and not taking action on this makes you complicit in this neglect which is child abuse.
Think about the bigger picture here. You don’t get a nice life with your newborn and hubs whilst you’re neglecting your stepdaughter- your a family, that’s not how it works. You’ll culture negative relationships, and DSD will know you don’t want her, she will start acting out, your life will be 500% harder.
Just get her some smellies when you’re next in the shop, chat to her, prompt her to do the washing And love her. It will reap great rewards both when she stops stinking, her confidence grows, when she remembers your kindness, when she grows into an adult who’s not fucked up by her childhood, when you build a relationship with her and stop feeling resentful and probably in a few years when you want a night out or weekend away she might even be a free babysitter.
Be kind op.