Regardless of what people think the mother 'should be able' to do, if she CAN'T, then that's fair enough. She's only human. And if she can't, she can't. We are all different and all cope differently. Just because you could cope with a big betrayal that tore the entire family apart and saw the children have limited contact with their father, for 20 years, doesn't mean OTHERS can. So stop dictating what you think the mother 'should be able' to do.
You know.....on this site when a woman comes on and explains her husband has cheated, has treated her and the children wrong, we all say that the kids will see through the father eventually with time.
It seems the adult children in this scenario HAVE seen through the father, and have seen for themselves what their mother went through.
And the deceased son, and the OP, have made their decisions based on that.
So.... make up your minds. Either the children will 'see and recognise the father's actions and treatment of them and and their mum', and will act accordingly, or not.
They have in this scenario. The deceased rarely saw his father and when he did, his hated SM was hovering around and he had no (according to the OP's posts) true father-son time. He saw the destruction that it did to to his main carer and main parent - his mother. So too, did the OP.
If they don't want the SM there, and neither does the mother, as almost the sole parent/siblings then that's how it should be.
I really wonder just how many on here that are soooo sympathetic to the SM and the dad would feel the same if some woman they detested tore their family apart, was part of the very reason for the deceased's destruction/death would want her there. I bet you anything they wouldn't. But they are unable to empathise or show any human compassion so don't have the ability to put themselves in the OP's shoes.