Hi, there
I have a step daughter who is 12. She came to live with us full time when she was 10, her choice and we later got a residency order.
I have always been quite a strong figure in her life and I was the one who noticed that when she used to come to us for contact, she would smell, have yellow teeth, and her hair was so greasy it was unbelievable! My Sd's mum said that my SD didn't like baths so she only made her have one once every few weeks. My SD came over one time, and decided she didn't want to leave. I'm the female in the house so it was up to me to tell her these things. So what I did wasI brought her loads of deoderant, roll on and body spray, all different fragriences so that she could choose what ones she liked the smell of. And I activly told her how to use them, i said something like 'now your getting older you need to us roll on and deoderant under your arm pits every day' I followed up afterwards with 'have you started using that roll on, i thought it smelt nice, did you?' I also told her that she was at an age that she has to wash regularily. Every other day or so. At first, I got blank looks, i know now that it was because she was depressed, it was change, and i was telling her these things that every girl/woman had to do, and it was confusing for her. Her mum never told her to do the simple things like wash often, use deoderant so she didn't know that such things existed. But I kept on, and kept repeating myself. I STILL have to show her how to wash her hair properly (she has very thick hair and can't get all the shampoo out sometimes).
Most importantly, me and my husband made it clear that she didn't have the choice, she HAD to wash, and keep herself clean.
when a child, like my SD doesn't know these basic things, it's really hard for them. If she starts washing at home and starts washing her own clothes, hr mum will question why, once your SD says that you and you DP told her she should, she will feel like she is being unloyal to her mum. Her mum does things a certain way, ie not washing, you do it another way, you wash. So your SD may fel like she has to choose between her mums way, and you and your DP's way.
Get your DP to phone her school and ask to speak to the school welfare officer/head of student services. You need to take a list of your concerns to the school, and ask their opinion on your SD's emotional welfare. See what they say. You also need to write a letter to you SD's mum, very important that this is done in letter/email form. Explain that you have found that SD is unclean and has poor personal hygiene and that these concerns need to be addressed. Your DP needs to stand up for your SD, it sounds like she's maybe depressed.
Has your DP offered to your SD to live with you? Making that change is actually a lot easier than people think, if your SD says yes she wants to live with you, then you can get the Child benefit changed over to you (It would be in your name, not your DP's as you will already receive Child benefit for your child together) once you have the child benefit number, you can claim for an increase in housing benefit/number of bedrooms in a council house.
I hope explaining my situation helps, if you want a chat then feel free to private message me :-)