No, you don't need to include their Modified AGIs.
According to Household income on page 3 of the Instructions for Form 8962, you have to include
the modified AGI of each individual whom you claim as a dependent and who is required to file an income tax return because his or her income meets the income tax return filing threshold. Household income does not include the modified AGI of those individuals whom you claim as dependents and who are filing a 2020 return only to claim a refund of withheld income tax or estimated tax.
Since you're not claiming your stepsons as dependents, their AGIs do not need to be included in the computation.
