If they were acquiring step-siblings, I think it would be clearer how to celebrate.
If you focus too much on celebrating this new sort-of-father-figure in their lives, you run the risk
of offending the actual father, who might feel that the celebration is about replacing him, or having a new family that doesn't include him.
Since they're both girls, why don't you focus on the girly aspects of wedding preparations? You and other key women involved in the wedding could take them to get their hair and nails done, or let their girlfriends come over for a slumber party and play dress-up, give manicures, braid hair and play wedding. With either approach, you could include a special dinner with their Mom and soon-to-be step-dad and say a prayer or give a toast focused on how they're all family now and there will be a lifetime of dinners together around this table... that sort of thing.
How nice of you, to think of this.
One woman in a house full of men: my soul mate:
... twin sons:
(HS seniors) ... step-son:
(a sophomore) ... our little man:
(a first grader) ... and there is another female in the house, after all