I am thankful for everyone's views but something like this I just have to correct. (It's my cousin and I love her after all). She is not "ready to walk over this because she is so concerned about future inheritances being inequitable."
No. It's about the fact that she feels he's putting Jane and her kids above - way above - his new family. That is what she's willing to walk over. It started as this issue and the more he resisted any kind of change or compromise the bigger it got for her - on an emotional level that something is not right here.
I wish John would be willing to try some of the compromises suggested here...but he won't