Survivor benefits increase when sibling ages out?

Sounds like your family was subject to the "family maximum" when you had two children receiving benefits. Here's the SSA's description of the family maximum :

There is a limit on the amount of benefits that can be paid each month on a person's earnings record. The purpose of this ceiling is to assure that a family will not get considerably more in benefits after a worker retires, becomes disabled, or dies than the family had in earnings when the worker was employed.

When benefits for a family would otherwise exceed the maximum amount payable, the benefits for all members, except the worker, are reduced proportionately to bring the total within the limit. As one child (auxiliary) turns age 18 and is no longer in a secondary school (high school), benefits to the other children will usually increase up to the family maximum. However, the remaining children on the record are each only entitled to no more than 50 percent of the number holder's benefit, and that percentage will not increase.

http://ssa-custhelp.ssa.gov/app/answers/detail/a_id/354

So in general, there is an expectation that the younger child's individual benefit will increase when another child "graduates" and no longer receives benefits.

However, if the child was already receiving 50 percent of the number holder's (decedent's) benefit, then the younger child would not see a benefit increase. This may happen if the decedent had a short work history of relatively low wages.

You may want to looking into an appeal :

http://www.ssa.gov/pubs/10041.html