Yes, I was wondering about that. I think it's more likely that she hadn't been told before, because I can't imagine that her first reaction would have been anything other than horror at the land being split (it was the lack of empathy or any kind of understanding that it might be a necessary evil or to offer any other solutions that made it bad for me, plus the petulant strop about moving out - presumably she meant that rather than suicide when she said she wouldn't be there anymore; I thought being initially upset by the thought of breaking up the farm was ok). So if her initial reaction had been shock and horror and this wasn't the first she'd heard of it, then David would have been much stronger than his ineffectual mumbling that of course she knew about it, and Ruth and Leonard would both chime in to say that she definitely already knew. And Leonard wouldn't have mentioned the land sale as though Jill probably knew, he would have been very sure that she did because she would have been talking about little else.
So ...
a) David thought he had told her previously but actually hadn't?
This is odd since she's regularly around, at least at meal times, and it's clearly a topic they've been openly discussing for days. Even if she's been going on day trips with Leonard since the fight (and I'm glad that there was clarity tonight that Leonard is no longer staying over in the guest room, by the way, I wasn't sure until tonight whether he'd ever gone home), the talk about the land sale started before then. So this feels off.
b) they all know full well that she has dementia and so weren't phased that she didn't remember the conversation?
It seems really unlikely that we haven't heard about this previously, and especially that it didn't come up between David and Ruth this week with David using it to excuse Jill's behaviour towards Ben.
c) Jill was told previously and took it well, and the family are surprised she didn't remember?
This seems unlikely on so many levels: every single person round that table was behaving weirdly if that's the case, for some of the reasons above. It's not inconceivable that they're only just starting to become aware that Jill's memory is going, but in that case they should have been much more surprised that she didn't remember, and all three of them insist on the fact that she did know. And I find it hard to believe that Jill ever took the news well.
d) David did tell Jill previously but she wasn't really paying attention and didn't take it in?
Still a ridiculous scenario, but possibly the least unlikely of the lot. Surely David would have noticed that she wasn't listening and checked that she understood, plus she would have had to not be listening on many different occasions when it was a discussion going on in front of her.
I can only assume it's a badly done introduction to dementia. Very poorly executed.