I'm on disability. I pay $100 out of my SSDI benefit to child support to my first wife for my 17 year old daughter, plus she gets a $222 benefit from Social Security. I get around $1,700 a month; my current wife gets $222 for her and $222 for our son and $222 for her daughter (my stepdaughter). Close to $2,400 for us and $322 to my ex-wife and daughter.
When my daughter turns 18 she comes off Social Security. What happens to that money? Also, if anything happens to my wife or stepdaughter (say a divorce or custody change), what happens then?
When your daughter turns 18, she will stop receiving money from Social Security. Your benefit will not go up, but your wife, son and stepdaughter's benefits could go up, because at that point there would be $888 to split between three people. This is because there is a "family maximum benefit," which is generally 150%-180% of your primary insurance amount, to be shared among a disabled individual and his or her family members.
Say your individual benefit is actually $1,776; your family members would have to split half of that, or $888. Your individual benefit and your dependents' benefits would total $2,664 ($1,776 + $888), or 150% of $1,776. (For more information on the calculations, see our article on Social Security's family maximum benefit.)
Now let's look at your second question. If your current wife were to become your ex-wife, your stepdaughter would stop receiving benefits due to the divorce. Your ex-wife would receive benefits as a divorced spouse only if you had been married for 10 years and she is over 61 or she is still taking care of your son and he is under 16. There are several possibilities here.
An important thing to remember is that whether or not your daughter or ex-wife or current wife or stepdaughter are receiving benefits has no effect on the amount of your benefits. You will never get more than you're getting now, except for annual cost of living increases.