I've been thinking about you all afternoon and the terrible circumstances you are in.
I think Starsky is right. Your wife (as the mother of your children) needs to know what is happening with your health.
Will it be a wake-up call for her? Most likely not (though for any sane absentee mother it would be). Still, in the interest of 'full disclosure' it seems like telling her is the right thing to do. That way she can't say she didn't know.
Your in-laws seem very supportive of you and the kids. Do they know yet about your diagnosis so that they can come forward with a helping hand? Perhaps they are the ones to tell first, and let them tell your wife.
Me: 35 Him: 43 Together: 19 1/2 years 1st Bomb (IDLYAM): March 2011 2nd Bomb (OW): April 2011 He abandons home/bills/everything: May 2011 He's bought a new house for OW: September 2011