I am sorry you are in such a sad situation. Just to give the other side of this, my husband is in a similar situation, but in reverse. His brothers really dont get on with him, and it has now reached a situation where they avoid him (we are regularly missed out in family dos) There is a lot more to this than I am writing, as it is not about us, just to give a different perspective. My FIL will be ninety next year, and as we live abroad, we have thought of suggesting that we visit before or after the event, to avoid making the brothers uncomfortable at a family get together, as they clearly dont want us around (it has been made crystal clear, in so many words)
Our suggestion is to make life easy for his parents, not for our convenience. Maybe your sons have not thought about how unhappy their never meeting makes you, and think that by having separate events, they are making your life easier? As you have gone along with this all these years, they may think it is for the best in your eyes. I dont know if i have made much sense - it is early morning and i am not fully awake - but I know what i mean!