I am not trying to be mean, but the post about getting tired of dealing with your SIL really rubbed me the wrong way. At most, you have been inconvenienced. Several years in a row, yes, but that sure beats what she and her husband have gone through. Honestly, it sounds like you just don't like her and would be annoyed no matter what. Because if I try to picture someone I adore going through the death of a baby and then another baby critically ill, I'd be doing whatever was easiest for THEM that year, no questions asked. I don't think you were wrong for making other plans on the other two weekends, as your life shouldn't stop for them, but then again I think I'd personally make allowances for them since the whole point of the get together is to have Christmas WITH them and other family. It's not like you were planning a random dinner party where they were just a few of the invited guests. The whole point was to exchange gifts with them so it makes sense to do it on a date where everyone can attend. I'd definitely cut some slack to a couple who didn't quite know when their baby would be healthy enough to attend. I'm sure if they had the choice they would choose to have two healthy babies and no reason to inconvenience anyone.
And I could definitely understand her comment about the food. The fact that she immediately was ok hosting after you declined doesn't scream "I expect you to provide the food!" to me, it says that she was, perhaps awkwardly, trying not to change your plans as much. Like she knew the change of venue was already inconvenient so she was trying to not make you change your plans for the food as well. The fact that she is perfectly willing to host means that she knows it's her responsibility at her home and is not trying to push that on you.
I am either missing a ton of backstory that makes this woman more evil than she appears or I'm just odd, because I don't see anything in this story that would really irritate me (and my husband tells me that I'm easily irritated all the time, lol). I see a slightly overprotective parent rethinking her initial choice to bring a sick baby out in the cold. I really can't blame her with the history of a dead baby. If that's the worst thing she ever does, it's not a big deal. Go and enjoy their hospitality.