1) The Brady Bunch is a lie.
2) It usually takes at least 5 years for step-families to bond, or become "blended".
3) There are LOTS of ups and downs along the way.
4) Teenagers are run by hormones - everything is high drama; so when something really IS high drama, watch out.
Pray a lot, be kind to the ex, be friends to the children, and be in charge of your own children's discipline for at least the next 5 years (even great stepfathers need time to become sensitive to the needs of the child)
I know one couple who established a 3 ring binder in the kitchen as "The Rule Book". Started out with the basic household rules, and then whenever someone did something not listed, they entered the new rule and punishment. That way no matter who incurred the infraction, the consequence was always the same, whether it was her, his or their child, and the discipline was handled by the natural parent.
Use the Wednesday nights as FHE; if some of the teenagers are not thrilled with church activity, focus on games and enjoyable time spent together as a family.
Wow, that's a lot (too much) to handle for a 15 year old. My compassion overfloweth. Talk with a counselor from rape crisis center on how you can best help her, and just be there for her, as you already have been. (It goes without saying she will be in psych counseling for a while.)
The best of luck to you! Stay strong and committed to one another! Remember to enjoy the journey!