@Wanderingowl you're forgetting that Mary already owns half of the estate though (because Robert made Matthew joint owners when he saved it via his inheritance from Lavinia's father in S3, and he then gave it to her in his will). They skate over the fact that for him to have been able to do this they must have broken the entail at that point (but basically the whole entail thing is completely misrepresented anyway as it's always been possible to break entails, and they were abolished in 1925).
So technically Mary could will her half to Caroline, so she and George could be joint owners. Or if George died in WWII he could will his half to either Mary or Caroline so both could end up owning it fully. However it's doubtful either would actually do that, as only George could inherit the title, and that was the whole point of primogeniture, that you wouldn't be a penniless peer, with a title but no land. But Mary could certainly give the majority of her inheritance to George while still making sure Caroline had, for example, that other Manor house they were going to move to at one point in S4, or the dower house, or the house in London or whatever.
I suppose the thinking was that the money in Sibby's new family all came through her step mother - although they all seem lovey dovey now there's no expectation a stepmother, particularly when/if she and Tom have their own kids, would want to give a fair proportion of her estate to a stepdaughter (particularly as depending on what the estate consists of, there may not be much to give away - if 4 kids own a quarter of a house each but they aren't allowed to sell it, it's as good as not owning anything!). Whereas Edith and Mary would always want their own daughters to have whatever they could legally give them, and both Downton and Brancaster are very big, well managed estates with multiple other properties.