Oh, OP, have oodles of sympathy 
My situation is quite similar.
When I met DP we both had kids lower end of secondary. In fact my dd was in Y6 at primary.
I have 2 dd’s, he has a ds and a dd. We were unable to blend (school logistics) and divided our time. Happy days. All compartmentalised. Then our kids started blending. His dd had crashed out of school prior to us meeting and wasn’t doing anything of note. Just at home and very helpless and sat at home drawing, pretending to write a book, nothing materialised. neither of her parents minded (fair do’s that’s their shout).
She is now 20 years old and doesn’t even do any laundry or chores. She barely leaves the house. She bakes cakes and makes scrap books and seeks validation from DP for all of the above. She won’t so much as get on public transport so DP is ‘stuck’ in his parenting, collecting, dropping off, shopping and cooking for them. All very ‘child like’. And frankly a bit bland. He wants to keep his kids coming to mine to wait on them hand and foot. I’m just ‘meh’ to all that now.
We (I) parked the idea of ever blending, so cringe inducing is his dd’s behaviour. And so incompatible is our parenting.
My dds are now 18 and 16, both have busy social lives, one is driving and working. The other about to go to college.
I’ve looked around at DP., as I look to the future and see my own horizons opening up. Reaping the rewards of parenting my kids towards richness in their own lives. What I see in DP is a man who seems stuck in ‘young childhood’ mode. Can’t say anything remotely negative to his dd. Pretends she is just ‘a bit immature’. They’re all in a sort of state of arrested development.
I’m ready to move on from needy children whether they be mine or anyone else’s. It’s a bit old and boring as we have been moving on. DP’s ds has been moving on. There’s a lot to lose as we have been together for 5.5 years. DP might still be parenting his dd like this when she is 40.
I’ll not be hanging around. I need to leave my DP behind to his permanent childminding and make a different path now.
to you