Yes, I'm the stepparent, we are the non-resident family and our finances are completely shared.
SD does not need wraparound care, she is a teenager now anyway, but her Mum is a TA and when she was younger I worked P/T and would have happily picked her up and provided care for her any time after school. Her Mum chose to send her to afterschool club once a week, but we made it clear that I could do it.
I'm not at all deluded and I'm very happy to show you the breakdown of costs for my DD if you'd like.
SD used to be with us 2-3 nights per week which meant we had most of the same essential costs as DD anyway (spare bedroom, clothing, school and clubs uniforms, toys, packed lunches) and also the same "luxury" costs (family holidays, days out, birthday and Christmas presents). The only real difference in costs were the fees for her extra-curricular activities which our maintenance covered more than the total amount of.
As she got older she wanted to be more with her friends and had more homework so didn't want to come to us as often, so we moved away to be near family, our maintenance went up as now it works out as around 1 night a week and we see her in school holidays and a few term time weekends, so in addition to maintenance we pay for her travel to and from where we now live.
It's quite rare for children to be in wraparound care 5 days a week and a little unfair tbh, 8am-6pm Mon-Fri is a very long week for a primary school child and leaves little to no family time or time for extra-curricular activities or just down time.