A new baby, in you case babies, is a change of circumstances and a legitimate reason to file for a modification.
The other-dependent deduction is part of an adjustment mechanism to apportion a parent's income to all legal dependents including those born before or after the children for whom support is being determined. Legal dependents include adopted or natural children of either parent who are under 18 years of age or over 18 years of age and still attending high school or other secondary school.
No adjustment is provided for stepchildren. Generally, stepchildren are considered the legal responsibility of their natural parents unless the court determines that a stepparent has a legal obligation to support the child.
Child Support Guidelines