I'm a stepmum and my stepchildern have stepchildren.
Once you start trying to "equalise" for what children might inherit from other parents, grandparents, step-parents or future spouses, it quickly becomes unmanageable in my experience and you leave yourself open to lots of heartache.
The biggest problem is that those future inheritances are uncertain. A parent might remarry, need expensive care, lose money, fall out with a child, or simply change their will. Basing your own estate planning on assumptions about someone else's future decisions can create a lot of complexity and, ironically, unfairness. And what do you intend to do....everytime someone loses money or has a fall out, are you going to re-write your Will to try rebalance things? It sounds way too impractical to me.
The question I'd ask is: what do you and your husband consider your family to be?* If you both regard all five children as your children, then a simple five-way split is easy to understand, easy to administer, and difficult to argue with later.
If, on the other hand, you each want primarily to provide for your own biological children, then splitting your estate conceptually into "your half" and "his half" and distributing from there is also perfectly reasonable. But you would need to own your house as tenants in common for this approach. But that can create very different outcomes, particularly if most of the wealth was built jointly during the marriage.
What often causes resentment isn't unequal amounts, it's children feeling that they were valued differently. DD2 and DD3 see your husband as Dad; if they received substantially less than some of the others because of biological links, they might understandably find that hurtful. Equally, his daughter might feel excluded if she is treated differently from the children who live in the household.
In my head, you have to appreciate that you cannot legislate for what other people may or may not give. You need to leave them out of the equation.
You can only decide what you want to happen with your assets. Everything else is speculation.
For a blended family, simplicity has a lot going for it. A clear will that reflects how you view the family today is usually better than an elaborate attempt to compensate for inheritances that may never materialise becasue it leaves your Will open to be considered as a reflection of the value your place on each of the beneficiaries of your Will and that could be very hurtful.. And what if you only leave little Johnny £10 because his bio Dad is minted and its expected he will inherit it all, and then bio Dad goes bankrupt, depends to live out his remaining years on a continual cruise or he remarries and before he sorts his finances, he pops it.
The real complexity is usually not the final split, but ensuring the surviving spouse is protected while also guaranteeing that all intended children ultimately inherit. Make sure you get your Will done professionally by a lawyer.
What anyone else may or may not leave them is none of your business and not within your control, its not a certainty and it may change.
I can't legislate for what other people may or may not give, they could lose the money they intend to leave them, to care home fees or they may sign up for a year long cruise and spend it all. One of the kids may land a really high paying job and another maybe on the breadline. I can't assume I will be around or have mental capacity to keep changing my Will to reflect other peoples life events.
I know that I want anything I have left to go to my stepkids and step grandkids fairly and what I leave is up to me. My money, my valuables, which I want to be left to reflect my feelings for them, with no favourites or consideration for other peoples financial promises. They only deviation from this would be perhaps Trust planning to protect anyone that is on benefit or is disabled.
Where I leave them 10p or 10 million each they all get the same because its a reflection of my love for them and not a "top-up" someone else's inheritance scheme.