Sorry, talk of treating her "equally" is really fucking weird.
In what way is being told it is now your job to pick a child up from school and provide childcare several hours a day about any kind of equality?
The woman having her time filled with work by other people is certainly not being treated equally.
You can't meaningfully treat a 6 year old and a newborn equally. It makes no fucking sense to demand that of anyone.
When I've had newborns the older children have had far more care from their father for a time because I was busy breastfeeding and providing the constant care that small babies need from their mothers.
And my older children didn't still have two other parents to care for them.
They had to deal with getting less attention from their own mother from a period and more from their father because the family had changed.
I left DD1 with her CM a couple of days when DD2 was born for continuity, to give her time away from the baby, and to give DD2 and me time together without a toddler setting the agenda.
I didn't use any childcare while on maternity leave with DD3 because I preferred to save the money. But no fucker told me that being off work with my baby suddenly made me responsible for all the childcare for all the children.
I am not a stepmother, but I find the demand that a woman who marries a man with children enter a fantasy world where she has to pretend that a child that has two living and involved parents is somehow magically her child too is bizarre and unreasonable.
And it's not even consistent because the "equality" demanded of the stepmother here is nothing that would, or could, ever be asked of anyone else.
The only way to make sure everything is "equal" between the older girl and Round's first baby is to go back in time and make sure the SD's mother had to do school runs on her maternity leave.
My children didn't have anything like equal access to me during my 3 maternity leaves - I took different lengths of time off, I used different childcare arrangements, I grew in confidence and experience as a mother.
This whole thing of a woman being told that as a woman/mother all childcare (and often housework) is now her job is about the opposite if equality. It is about women being treated as drudges.
For the childcare foisted on a first time mother without discussion to be for a child that has two parents to look after her appals me.
Should my daughters ever have a stepmother, I would be horrified to find out their father was treating his new partner as a skivvy in this way.
It's fucking horrible.