I'm coming to the thread late and hence have the benefit of all your posts, OP. Well done for extricating yourself from the relationship with ex and his family, and doing so with your head held high and leaving relationships at least possible for your DC. Based on all you've shared, that is some achievement.
One of my parents was a Wills & Probate solicitor. I'm not qualified at all, but working for them for a long time gave me some insight. A lot of people have understandably speculated that the Will was just 'written badly' and should've been written to include the younger cousin. I'd have to say that it seems unlikely given the Will has been written relatively recently. Allowing for the birth of more children/ grandchildren in the future is pretty standard stuff with Wills. It seems far more likely that the deceased made very deliberate decisions about what they wanted to happen to their money when they died and given the nightmare that family, particularly some parts of it!, is, it's not hugely surprising.
But whatever the position, as many have said, the Will represents the wishes of the deceased and should be followed. I can't help feeling somewhat sorry for the cousin since at 10/11 they have clearly done 'nothing wrong' yet there is a case to say they've missed out on a sum of money that is, realistically, life changing given it means a different/ better education/ deposit on a house/ whatever (if spent like that, anyway, rather than frittered away). However that is not your concern and given the deceased was of sound mind when they made the Will (which you have to assume they were if a professional was involved) then they have made their decision and you have to trust they knew what they were doing and had their reasons.
What I can say is that my parent, when writing somebody's WiIll, in a situation where they could be perceived to be 'disinheriting' somebody. would often suggest they write a side letter ("Letter of Wishes") alongside the Will. In this case they could've written a letter explicitly addressing the reasons why your DC were given an inheritance and the cousin wasn't (e.g. "I always admired the way IfIHadAHeart conducted herself throughout the relationship and divorce and having become well acquainted with the DC I would love to help them out financially, whereas DC's Cousin is well provided for already"... or whatever the rationale actually was). That way it doesn't become part of the publicly available record but it does provide a contemporaneous record of the deceased's wishes and rationale, thus reducing the risk of their wishes being overturned in the future. Of course, that does rather depend on executors a) receiving the letter and b) not 'burying' it. If solicitors are named as executors or co-executors that's one thing but if only executors are part of the Nightmare Family from Hell, them executing their legal obligation may not be reliable, let alone taking into account their purely moral obligation to consider anything in a letter of wishes!
Anyway the whole thing sounds like by the time it's over, OP, you might be willing to pay £100k (or whatever the actual amount turns out to be) to be rid of the lot of them..! I hope things calm down soon, that your DC get enough money to make a real difference to them and that there isn't too much collateral damage.
And no... you're not being unreasonable. One of your posts talked about what 'fairness' is in this situation, and you're absolutely right - so much depends on context. I can imagine many scenarios where splitting the inheritance 3 ways instead of 2 and going for a Deed of Variation to achieve that would be the 'fair' way to go (although maybe not even possible in this case). I can think of others, as PP have explained, where the generation up collaborating to achieve an equal outcome for their children is 'fair'. But 'equitable' and 'equal' are not the same things and it may well be that the deceased saw a way to make things equitable in ways that you are wholly unaware of. You and your DC have no reason to feel guilty and those who suggest the only fair way forward is a 3 way split have very little imagination! (and to be fair, in some cases, also didn't have the benefit of the information in some of your later posts when they replied).