YWBU yes and I speak as someone who would be in the position of your DD1.
In my situation we are 4 siblings.
I am the oldest, my mother was a teenager when I was born and got pregnant with my DB1 when I was 6 months old, so I never got the financial benefit of being an only child, nor the one of having financially stable parents as apart from my mother being a child my father got out of the picture very early on and is the type to live off other people's money so there will be no money to his name at his death for sure.
DSB2 and 3 were born a few years later from my mother and stepdad, who has no family.
DSIS is not born yet but should be soon and his from my mother and her new husband who has a lot of family.
So while DB1 and I were born in similar situations with a similar upbringing, I moved out at 16 and have been paying my way ever since, while DB is in his 20's and still living at home rent free with his clothes, food and phone paid for.
In the meantime DB2 and DB3 knew a more financially stable environment growing up and while my mother and their father are modest, they may still get something from him (though likely very little), and like me and DB1 have no paternal family to inherit from.
Dsis on the other hand, who is not yet born, is going to be the best one of, apart from one my siblings are over 18 and should be moving out soon so she will have the advantage of growing up an only child (so more access to money from her parents+ extended family) and also have lots of family to inherit from.
My mother have nothing to her name either but according to you, once she inherit from my grandparents she should.
Split it in my favour most because I have had the toughest situation of the 4 and the least access to money throughout my entire lifetime. Then give DB1 slightly more than DB2 and 3 because they might inherit from their father when me and DB1 won't and she should cut off the baby because she will obviously have a much better start at life and will continue to be privileged throughout her entire lifetime.
Even though your technic would benefit me, I don't think that's fair.
At the end of the day even though my mother technically screw me over money-wise by deciding to have a child at 17 with a jerk of a man, it's not fair for my sibling to be desinherited.
I think heritage always causes rife and will cause rife regardless, because no matter what you do, your youngest one will have more than your oldest ones and their step-sister will be a millionaire whether she inherits from you or not and it would be unfair for her dad to cut-her off from an emotional point of view and it would cause much more pain and damage to her at a moment of her life where she just lost her father than not getting the money would for your kids (and I say this as someone who will be cut off her parents wills and don't care).
I definitely know no matter how my mother's will is written (I am no contact with her so I won't be on it) my youngest siblings will resent the fact that our upcoming sister already starts with an advantage, the same way I resent having had to fend for myself since I am 16 while DB1 is still living rent free and bill free in his 20's.
No matter what she does, split it fairly or unfairly, she won't be able to fix the feeling of unfairness and difference in treatment that almost automatically comes with the choices one makes to have different children with people with different economical background leading to very different upbringing and life opportunities.
My grandparents have three daughters, my mother and my aunts, my mother is very modest and owns nothing while my aunts are both millionaires with several properties and businesses etc... my mother would need the money most but they've already said that they will split the money fairly and that's only right. My mother made life choices that led her to not become a millionaire like her sisters did, my aunts shouldn't be deshinerited because my mother didn't make the right choices. In your case, you made choices that means that no matter what your kids won't have the same economical security, it sucks for them but it's life and they need to accept it. Nobody is owned an inheritance to start with so whatever they get is bonus.