I understand that you need to make provision for a disabled child, but I do hope that once your other DD is over 18 that each of the three non-disabled children will be treated equally, irrespective of who will or won't be inheriting money from other relatives. That is besides the point.
I only have one DD, so not sure where you got "your other DD" from.
And it's NOT "besides the point." It's PRECISELY the point. Once DS2 reaches 18, it will be adjusted, with provisions made for university fees and such, and amounts altered accordingly, provided he plans on going to university. If not, then another adjustment will be made from there, based on our family situation at that time and the outlook for the future.
Our children realise that any money left is not "reward money" for being our children to be doled out equally, but money to make sure they all have the opportunity to be financially stable first and foremost, with money to help if needed and available.
A lot of things can change in 15 years, and we will make adjustments that are needed when we reach that point.
As I said previously, my parents have their wills stating that the money is to be distributed equally amongst their grandchildren, instead of to their children. This is because one of my siblings has stolen thousands from them and would not use any inheritance other than to blow through it with no benefit to her children. So to make things "fair" to everyone, they settled on the grandchildren directly, as most of us are financially okay anyway. Whilst there was a bit of annoyance that we were passed over because of the actions of my sister, that annoyance was dismissed pretty quickly on my part. My parents have a right to leave their money to whoever (whomever?) they want, without interference from me or my siblings. And to be honest, I'd have used any inheritance to directly benefit my children anyway, so this just cuts out the middleman.
My siblings are not thrilled about it (understatement), however that's too bad. They wouldn't be happy no matter what as they're quite grabby and selfish - having all started demanding money and items from my mother right after my father died a few years ago. 