Serkeen, I do hope you find yourself getting stronger as time goes on, and your son is less present in your life.
There are clearly many difficult aspects to this whole situation, perhaps going back to your son's childhood.
Your ex sounds weak and unkind, and you sound as if you feel responsible for the whole thing, or at least responsible for solving it, but of course this cannot be the case. Both your ex and your son are adults, they have agency, and the power of choice, and for whatever reason they have chosen to behave badly and cruelly.
It might help you to have some talking therapy, to work out how best to gain perspective and to preserve your own mental health. You can't change how your son behaves, but you can change how you respond to him.
Of course your sister is 100 per cent wrong! It's not a 'generational thing' at all. Many of us here have grown up children, and we would not expect to be treated badly, and to have these unpleasant, hurtful things done and said to us. My adult kids are not perfect at all but they would never ever hurt me like this. You are right to feel let down and disappointed by what he is doing.
One thing: I presume you are no longer supporting his family financially? That would be a good thing to stop now. If you have spare money for them, perhaps put it into account for the grandchildren.