There are too many unanswered questions here.
What were the circumstances of the divorce? Marital abuse or amicable separation? Has the step father been abusive? Are the brothers unreasonable jerks who just want their mommy & daddy back together again? What does Momma think about this? Why isn’t the girls biological parents paying for the wedding?
Hard to tell who is wrong here, however, if I was the step father & was disinvited to the wedding, my checkbook would be closed immediately. Let the brothers pay for a wedding for which they have dictated the guest list.
“ Why [aren’t] the girls biological parents paying for the wedding?”
Because, as generally is the case, the step father is a sucker being drained of his money and used.