Disability and Children

I am disabled.
We were together for 13 years, I never wanted kids and she said the same thing, then an illness I have always had kicked-in and I started getting very ill.
All the sudden she wanted to have a kid. I thought it was odd... six months of therapy later "we" decided to do it and I am happy with my son and life.
Though it is a real challenge, especially now as my condition(s) accelerate.
As this happens and my pain and my future becomes uncertain all we discussed in therapy (just one!) is out the window and she wants to try again... Now that I can help even less!
By all evidence over the last 16 years my wife is an intelligent person, logical and has always looked at the world with logic and clarity.
I think she is losing it.
Don't get me wrong, I love my son, I love my family and the arrangement we have and would not trade him or this for anything.
But, I suspect her reasons... is she trying to "get as much of me" as possible before the inevitable happens?
Is this just some baby-hobby all the sudden.... Grasping at youth, disregarding the reality of our situation? Combination? No one but myself knows about her new desire for "a daughter"... I guess we get to pick... though we will be trying the timed method for how fast the sexed sperm swim. Does help, I understand.
More than anything I feel bad because I won't be able to help at all with the baby this time... I will be left-out and I don't know how to deal with that, and I also know how hard that will be on her as well... a lot of issues.
I also feel very selfish in feeling trepidation for what happens when I have a bad pain day with an infant in the house? Not much help left for me... I feel like a real shit-heel thinking that way.