An infant is an infant, they are barely even counted as a passenger when they don't have their own seat. Totally separate thing altogether. The only reason that they really do that is because the FAA lets them (the 2-year limit is an FAA thing, not United or any specific airline). Parents in law may not be blood-related, but they can still be considered close family members. And I highly doubt that anyone in the employee travel center would change the policy because an employee's sister wants their kids to be able to fly, no offense.