@Airworld The first thing I want to say is that I'm sorry for your loss. This must be an unbelievably difficult time and I hope you're OK. Your feelings are natural and understandable, whether there was a good relationship or not. You feel what you feel and that's okay. It 's a process and it will take time, but it will not always feel or be like this. You're in the trenches right now.
Second I wanted to say, in relation to your friends, that most of us don't know how to deal with grief. We don't know what to do or say or whether or say anything at all. We laugh now at the Victorians and the big fuss they made over death and grieving, and the social rules they had, but TBH I think they were on to something. So some of it may be just that rather than anything personal.
Not going to see her/the funeral - I don't think it's that unusual, TBH. It doesn't make you a bad person. As for the flying monkeys swooping in, they are grieving too, and it doesn't tend to bring out the best in people. The fact that they're behaving like flying monkeys even now suggests that they had been influenced by narc behaviour, so there may have been more of it displayed than you think.
Please contact your SIL. She's reached out. Reach back (otherwise you're at risk of doing what you've said other people are doing to you).
And finally this:
when the other person dies people think you are disgusting/ don’t bother with you anymore.
Is this your mother talking? The little voice in your head telling you other people think you are disgusting? You don't know what they really think. I know it's difficult, but it's important not to project/assume. Something happens with a narc parent (I know because I went through this) in which the hyperfocus on what they say are your faults makes you feel like those faults are so obvious that everyone else in the world must hate you for them too. You become very internally focused and self-conscious. But here's the thing. You are just not that important to other people. They are too busy worrying about their own shit to notice other people half the time.
Block people who are making it worse and reach out to the offer of help. And you've got freedom to look forward to now. The relationship is finally done. You can breathe.