Kristne, child support laws vary by state. Best advice is get an attorney. In Texas, depending on the wording on the divorce papers, the combined incomes can be figured to determine child support. In my case, only his income was considered. If other was considered, I'd have gotten a bigger check. However, because of wording on the papers, my aunt's income was used in conjunction with her husband's to figure child support paid for his two children.
As far as I know, unless you adopt the child, you are not responsible for paying his child support fees at his death. (However, if you and your child receive death benefits, the other child likely will receive a death benefit as well, depending on how that's set up, you may be the person assigned to oversee the amount on the stepkid's behalf, and thus responsible for making sure a percent goes to stepkid.)
And as far a I know, the amount does not change just because he had another kid, however I believe there is a stipulation that the amount a parent pays cannot exceed a certain portion of the parent's income. That's why some dads who have children with multiple women are required to a bunch of support, and get way behind.
There usually is a time frame for which the support issue can be reviewed, generally 2-3 I think.
Again, your best source of information is an attorney. Review custody papers, and have them ready when talking to people who know. If in Texas, you might check with the AG's office. They have a division dedicated to child support, and might be able to answer basic questions, although keep in mind they do not represent either party legally, so questions may have to be generic.