What concerns me most in your letter is that this situation continues to upset you to the point that you leave family events in tears. That suggests that you are still hanging onto some hope that the family is going to change their treatment of you. You do know that they are not going to change.
I think there is more than jealousy going on. The fact that you have had a successful life makes your siblings feel bad about the choices they’ve made. If you are successful, it means that maybe they could have been as well. Rather than look at their own inadequacies, they create a narrative that makes you the person at fault. I have a guess that they see the potential for inheritance as a way to bail themselves out of their situations. They may also feel that you are so successful that you don’t need it. If they cut you out, there is more to go around.
My advice? Focus on what you really want, which is to have no regrets about not seeing your mother and to have some time with the cousins. Limit the time at family gatherings to spending a few minutes with your mother. Make a coffee date with the cousins while they are in town. (If they don’t take you up on it, accept that they are less invested in contact than you are.) Then leave. Don’t stick around to be bullied and put down.
Instead, focus on celebrating your own family. Stay in touch with your adult kids. Have good times with your husband. Enjoy the family you’ve made, instead of trying to change the one you were born to.
If the situation continues to bring you to tears, please see a counselor for a few sessions to help you learn new ways to not let them get to you.
I wish you well.