I think it's fairly easy to challenge that sort of thing - it's usual to treat gifts given during life as being completely separate from inheritance for tax purposes, and the other side of that coin is that the courts will say that all children should inherit equally, never mind what gifts had been given previously.She'd likely owe 000s to the estate . . .
The only way to get round it would be using trusts and suchlike, but I don't think Big D's estate is big enough for that to be worthwhile.
So, sadly, Jack's likely to inherit the same amount as her siblings, no matter how much extra help she gets given whilst her parents are still living.