I just read the post "Christmas question"....
We had a similar situation in that our income is disproportionate.
Well, I don't have a black and white answer, but here are my thoughts.
The main thing that whether you are talking about a pre-nup or how to manage finances now, it has to be something you can both live with and not harbor any resentments over.
I think it will be easier if you don't combine the 2 separate issues of splitting expenses now and what would happen to your assets if you got divorced or died?
First I think it's totally fair to split expenses in proportion to income--you aren't really being a burden on him if he was able to pay for all his household expenses by himself before you & your kids moved in.
That is exactly what dh did before we bought our condo...took out a life insurance policy on me to cover my share of the condo which, because we are now married, own jointly.
You're situation sounds perfect!