See, I have no issue whatsoever with the mother's death being removed from TSP and inserted into TWS, though I wouldn't have pretended it was happening in the 'now' of the timeline, I'd have done it as flashbacks. Though I suppose there was no 'now' of TSW's timeline, given that it purports to carry straight on from the end of TSP, but doesn't, and none of the approximate dates given bear any resemblance to actual events we have dates for eg the Icelandic walk appearing to have happened at an entirely different time period.
I have no issue at all with SW's sister not being mentioned at their mother's deathbed, but I would have assumed, all other things being equal, that she was omitted because she'd asked to be, which isn't unusual in memoirs. A friend's memoir about a year spent living somewhere unusual gives the impression she's a single parent to a young child, because her teenager and DH didn't want to be in it, and she respected that. I visited her at one point and was with her during one of the more dramatic interludes, but asked to be left out, too.
But given what we know about SW as compulsive liar, fantasist and thief from family, it's perfectly possible that she's deleted her sister because her sister's presence just called up too much difficult real-life stuff, about family 'rifts' and the reasons for them etc etc.
Though obviously it's not entirely clear where SW's sister sat in the 'rift'. Obviously discovering your sister had stolen your elderly mother's entire savings would be a fairly understandable reason for not being on good terms, and the typed confession appears to have been addressed to her, but her daughter was the one who gave the Walkers a free place to stay for eighteen months... ?