OP I'm not sure why you're getting so much stick here. People seem to forget how stressful it is when you're faced with being in labour for the first time.
There are two things here that make this different to 'having DC1 around'. One, you, by definition, have never given birth before, and it's totally reasonable to be scared or stressed.
Two, when it's not a child you live with all the time, it can be difficult to predict how they will react to a given situation.
I have a DSS (11) and I wouldn't have been too keen to have had him round whilst I was in early labour as he can be hard work. As it happened, his parents have a very flexible approach to access so it wasn't an issue - and his mum would have understood if we'd had to change plans.
However, if he had been here, we would have managed fine until one of his grandparents came round to look after him or take him home (he lives 35 miles away).
In fact, having your DSD round might help you stay calm (in that artificial hidey feelings way that all mums/stepmums do) and give you something to focus on.
It seems like you have a good plan in place so it will be fine.
I do understand your concerns though. Especially as you're hormonal and worried about giving birth.
As a side note - someone mentioned upthread about being able to introduce your baby to ger sister... This is a nice moment (tho nervewracking) but at the hospital I was at, this wasn't allowed. Only the mother's own children are allowed on the postnatal wadd due to infection control.
Good luck