Hi OP! If you’re posting in a stepmum type capacity on here you’re going to get a variety of scorn and flippant comments - don’t take them to heart. Put them to one side. In this case it seems to have focussed on the example of DPs daughter calling him daddy at 11 yo in your request for help/advice.
The youngest is 11, so very soon she will be a teenager. Average teenagers loose interest in going out and doing things with their parents very quickly, so for you it could be a matter of waiting it out. Try to make the most of this time as much as possible to bond and make memories. In 4 years time when she comes to your place and sits in her room on the phone the whole time you may miss her enthusiasm for doing things (or it may be a relief and you can all quietly get on with doing what you want). I’m assuming she doesn’t live with you so it sounds like she values the finite time with her dad (and yourself!) and wants to make to make the most of it.
You have a good relationship with DP, you don’t want kids, so him already having kids works because you won’t feel like you’re taking the opportunity from him. I don’t think there’s any problem with this. As long as don’t obstruct his relationship with his kids, initiate any conflict with their mum and try to get on with kids as best as possible and there’s no nastiness, all will be fine. You don’t need to be super mum - they already have a mother. If there’s no issues between DH, you and their mum (you don’t mention that there is) this is a massive boost for the relationship.
It doesn’t sound like it from your post, which comes across as light hearted, if there are any deeper issues that you weren’t able to comment on your post that are causing you greater mental suffering you need to leave. I don’t know how long you have been with DP but if it isn’t long really think long and hard about this.
90% of the time blended families are extremely hard work and painful. 99% of the time they are not for people that want an easy life.
My story is different to yours and I’m not telling it for you or other posters to make direct comparisons to. Just to give you an idea of my experience for offering above advice. I was 27 when I got with now DH who was 31 with DSS 7 and DSD 10 yo at the time. I thought it was going to be easier than it has been. I looked forward to meeting them and wanted to spend loads of time doing activities together.
DH had been split from their mum for three years at the time and I wasn’t involved in that. There was conflict between DH and her, him getting into a relationship with me seemed to fan the flames (I don’t know why as she already had another baby daughter at this stage - maybe stressed out with the baby?). It hasn’t really got much better 10 years on. I thought it would. If I could go back, me now would tell me then to leave because of the ex wife involved in this situation. It’s caused a lot of mental pain and I have questioned if I wanted kids myself because of it (side question: are you sure the issues with step kids aren’t putting you off having kids or didn’t you want them before?).
Silver lining is that the kids are great. Due to the stress with trying to arrange visits they were usually apathetic by the time they got to us and didn’t want to do much. But we did do things and had a some nice times.
They’re older now so they can see the whole picture and are good judges at what’s going on. I don’t know where she’s got it from, school friends maybe, but DSD 17 has a strong sense of social justice and won’t tolerate unfairness toward any of the parties involved (even, shock horror, if it’s directed at me). Wish things had been easier with their mum when they were younger so we could’ve made more memories, but starting to see the light at the end of the tunnel - step kids are growing up into reasonable adults and our involvement with their mum will soon be very minimal.