I am all for discussing it. However, the wife and the son cannot understand what it is like to live with an abusive parent, being helpless in a situation to see one parent abuse the other. Unless they have lived through it, and even then not two people are alike.
To me the OPs feelings are worth more than his wife and son. In these situations it easy to get talked into something, as your emotions are all over the place. If the wife cannot understand the point of view of the OP and only wants to go because that's what you do, or because the son wants to go, that's not a good argument.
For those who say 'for who would you go'. Even in these circumstances you can go, with no one to talk to, it can give closure for yourself. There is something definitive at being at the funeral.
To be clear, I think you should go,
She will understand.His current wife is amazing and it would be for her that I would go.
I think you should not go to the funeral and enjoy the trip. If you or the 8 year old feel the need for some closure, you can have your own little ceremony or memorial somewhere along your trip.
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I'm writing this from Mitchell, SD. As of this morning I still had heard nothing so we packed up the car and left for our vacation. About 6 hours into the drive I received a text from my step mother informing me there was no funeral, family only was invited to the burial in the family plot on the reservation followed by a feast. We sent our apologies that we could not make it as we had a non-refundable vacation planned that we had already left on. I told her I would get together with her when I come into her town in a couple weeks. She seemed ok with it.