The problem is that your family is influenced by a positive motive rather than negative. They think that by shunning you they will save your life. They believe that whoever rejects "Jehovah's organisation" is already doomed. That positive motive—saving your life—is a big problem. Negative motives aren't as effective.
Your family also sincerely believes that if you are outside the organisation, you are more likely to become an immoral person. Again, they think that they are helping you by shunning. In the case of the rank and file members of the organisation, their belief is rather sincere. It's the leaders who are really selfish.
Hence, what you're fighting against is not simply shunning. It's the belief of your family that they are saving your life—and theirs by adhering to the rules of their leaders. You need to show them that their leaders are wrong. However, that is nearly impossible. You were a Jehovah's Witness, so you probably know why it is so hard.
Proving them wrong is not a short process. It takes a long time—even years. Possibly, they will acknowledge your happiness even though you are not in the organisation. However, that raises another question. Will that matter? What if your family does believe—as many Witnesses that I have seen myself—that it is possible to be good without being in the organisation?
Moreover, when it comes to your mentally disabled brother, I assume he is being taken care of by your family as well, which would suggest that he is also taken to the Kingdom Hall by them. If such is the case, either you will have to convince your family that their leaders are wrong, or you will have to think of a plan to associate with him—however briefly—without their knowledge.
To me, it seems like another case proving that the Watchtower can ruin family life. I wish you all the best. You are in a hard situation. You will have to endure and lead the happy life you say you are committed to having. I think you may have to accept that you may not see your brother in a long time. Nonetheless, this is not a definite statement. It may be the case, though I wish it won't be.