Op, you are taking a lot of crap on here. Ignore it. This is the steps board,where a strange universe exists whereby you must treat your step child as your own, put said child above your children and yourself, but also understand that whilst doing so you will be vilified for "over stepping". You can't win, and you don't need to justify yourself to anyone on here.
Fwiw, I was a fulltime step mum to one child, and a part time step mum to another. I had a dd of my own, and me and Dp have a dd of our own. Sad to say, my Dp was very much in the "taking the piss" category, and always felt I was free child care for his children. He went to work for very long hours, and insisted on his full entitlement to see his children, and saw no reason why I would mind looking after them eow and half the holidays.
He had his children for 3 weeks last summer, when I had a 3 week old newborn, and my dd off school (she was 5). He did not take one day off. I was trying to breastfeed and there was no where I could go and just be in peace.
I tell you this, because it is easy to fall into a trap of being pushed into these things, and it becomes the thin edge of the line, because once you have done it once, it's hard to say no next time. If you don't feel you can do it, then don't.
My baby was (and is) a nightmare sleeper and very full on. After 3 weeks with all four children here and no help from Dp I was a physical wreck. I did put my 5 year old dd into a play scheme because it was really hard (just shows you, if it can be done for my dd there's no reason why your dsd can't stay with her childminder too)
I still look back on that period with resentment, that no one realised how hard things were for me.
If I had this time again, I would not be pushed into what I wasn't comfy with. This is your first baby, it wasn't for me and I struggled to do it all.
It won't kill dsd to stay in her childcare and she won't feel pushed out. And trust me, a 5 year old and a newborn can really be very wearing together 
I hope it goes well for you. Take the good advice on here, and ignore those who frankly know sod all about these things. It's very easy for people to spout how it should be, while they do not live a step parents life.
Good luck x