Oh bluebell that's awful to read.
I too have a disinterested and unsupportive mother, who, over many many years has barely given me any kind of support when I've genuinely needed it; be it emotional - which is what I crave most and am most upset by, practical or financial - I don't expect the last two BTW but also, like you, I have a sibling who's a nasty piece of work for all sorts of reasons and who my mother has bent over backwards for to a ridiculous degree, including huge amounts of (free) childcare and 'loans' which are never repaid.
All that is bad enough but thankfully, I have never had to face the trauma of losing someone I love like you have and can only start to imagine how much more shocking and hurtful it is for you - and your children - to be rejected by her even in such extreme circumstances where most normal compassionate people would be desperate to help you however they could and actively watching out for you. The fact she hasn't done this and is trivialising what's happened must really shake you to the core.
I think the overriding notion of 'mothers' in society is that they are caring, nurturing figures. When you have one who isn't it's extremely hard to get your head round and many people in that position will question what's wrong with them first before looking at their mother's failings. It really messes with your self esteem because your mother is supposed to be the constant person in your life who'll always care about you and who you can always turn to. I think if you have siblings who are treated differently your esteem takes even more of a battering as it demonstrates that she is 'at least' capable of showing care, love, support, whatever to someone - as opposed, say, to being completely cold emotionally, which would still be bad but not as bad as feeling you've been singled out for some unknown reason.
Like you going NC isn't something I've managed yet - I guess because you cling on to the hope that 'one day' things might change and/or you might be able to 'get through' somehow. I know I'm deluding myself about that although I have gone NC with the sibling. I do keep a large distance from my mother however - am polite when I need to be - and acknowledge birthdays, Xmas ..... but no longer try to engage her in any sort of emotional chat, or tell her about any worries, or problems I have as I know I'll be rejected (favourite is quite literally making an excuse if on phone within 2 mins to end the call and then I hear nothing, despite having just confided something quite upsetting, for months on end) and it's come to feel as if I'm setting myself up for a fall. My mother will quite happily talk about my sibling (FFS), her neighbours, TV, the weather, and herself and quite frankly I'm not sure I could have a fulfilling relationship with anyone whose interaction with me was limited to more or less those areas so now I only speak to or call her very infrequently - she probably calls me twice a year! Protecting myself by withdrawing doesn't mean I'm 'happy' about the situation - at all - but it's 'better' than subjecting myself - in real time - to the sure and certain knowledge that she doesn't have any interest in me. I know she doesn't of course whether I speak to her or not but somehow having it confirmed yet again just rubs salt in the wound.
Maybe if NC is too much - you might consider doing similar ? ..... pulling right back and not allowing her to rub your face in her indifference (and, quite frankly, shameful lack of compassion given your loss) by giving her the opportunity to say something else rude/tactless/irrelevant/unfair all over again.
I do think there's still quite a taboo over going NC with a parent - it's beyond the comprehension of those who have normal loving relationships. And it's a big step to taken when some people are so ready to judge ... keeping her at arm's length though perhaps wouldn't seem so final and wouldn't be something you'd have to flag to the outside world. You certainly shouldn't feel bad if you do that - you need to conserve your emotional reserves as much as you can, and not waste them on anyone who clearly doesn't give a stuff about you.