I am not sure why you are so keen to favour dc1?
Why is it so important to you that dc1 should inherit two dads, one bio dad and one step dad and dc2 only ONE dad, when it would be more fair that each child inherit from their biological dad?
Especially when you know that dc1 will inherit her bio dad, then 50% of your share, why do you want her to share your husbands 50% with your husbands biological child?
This is what you seem to want:
DC1
100% of bio dads large estate
50% of mums estate
50% of step dads estate
DC2
50% of mums estate
50% of dads estate
You seem to be really keen to find ways of favouring your eldest.
Is that reflect in day to day life with your two children also?
Is it really so bad if the following happens?
DC1
100% of bio dads large estate
50% of mums estate
DC2
50% of mums estate
100% of dads estate (which is still just 50% of the family estate)
Then you have two children inheriting 100% from their dads, and 50% from their mum
DC2 is still going to inherit a lot less than dc1 as DCs dads estate is inherited in entirety.
In real terms, dc1 inherits 25% of the family unit estate, and 100% of her dads, and dc2 inherits 75% of the family unit estate.
This is not unfair.
Why do you want dc1 to inherit so much more than dc2?
It seems to me through the posts that you are not keen to make it even, but keen to justify why dc1 should have more. Sorry, that is just how I see it. And you dont need to answer to me why you favour one child so much, but think through this yourself.