The fact that no one would actually really care if she bought a couple of cartons of eggnog from the store tells me something interesting; what it tells me is that the family that allegedly doesn't notice or appreciate Elly's hard work actually stands around scratching their heads and asking themselves "Why does she do all of this stuff anyway? It'd still be Christmas if she didn't make eggnog or all the other busywork so why does she act as if the world would end if she didn't?"
That being said, you can't tell her that she could trim her to-do list with the same ease she does her Christmas card list. After all, the problem with explaining that she could not do things and have some peace of mind is that she doesn't want to know that she can make her life easy because she's the sort of restless idiot who can't leave things be. Just as John's wondering how it could take her all day to clean a house that his mother could tidy in half an hour was misconstrued as being an accusation that she's lazy, telling her to sit down and take some time to herself will inevitably be misheard as an accusation that she wants to hibernate.
What really, of course, seals the deal is that she doesn't know this about herself; I could tolerate her inability to do nothing a lot better if she were at least honest about it. The trouble is that she thinks that she's supposed to relax despite the fact that the few times that she allows herself to, she hates it with a passion because there's always something that she needs to do.