I think that I am blessed with the most crazy, mixed up, step-parent/children relationships around.
I married my husband nearly four years ago, having known him for 7 years at that point. Between us, we have eight children: my two daughters, his two daughters, and his four step-children from his second marriage. Five girls and three boys, altogether.
My daughters knew him well before he became their step-father, because we worked at the same place for around four years. He was still married to #2 at the time. His natural daughters had never met me, but I had seen #2 and all the kids many times. When he told his daughters that he was marrying me, I think I had to undergo some kind of test. Neither of his sisters nor his daughters liked #2. We had moved in together a couple of months when his oldest sister and her husband made a trip in their 5th wheel to see us. I was pretty sure I was being scoped out for the rest of the family, but I seemed to pass the test OK. Then, his younger sister came out for our wedding. As a surprise. I knew when she came into the room where I was dressing and took my picture while I was in my underwear I was really being tested! I just laughed right along, of course! (Nobody ever died of embarrassment, they only wish they had!) I passed that one with flying colors!
A couple months later, we took a trip to see his youngest daughter in Washington state. She had had her first baby soon after we were married. We talked a while, then she dumped Katelynn in my arms. I figured that one was a go, as well! I had to first meet his older daughter by a telephone conversation. She asked me to describe myself. I said, “I’m a short little round woman.” She laughed and then asked me who my favorite Jedi was (hubby had told her that I’m a Star Wars fan deluxe). When I answered “Qui-Gon Jinn” she said I passed her test. 
However, they really decided to test me the following year: his older daughter was flying out from Florida with her two boys to her mother’s place in Arizona, then was borrowing a car from her mom and driving up to stay with us in Colorado for a week! Her sister was so thrilled, she decided to come down with her husband and baby from from Walla Walla for a visit, also. :eek:
Did I mention that my house is a duplex with just over 1000 square feet? And that the couch we owned at the time would only seat 3 people, had a back leg that collapsed regularly, and was reinforced under the cushions with plywood? And that my dining room table was a 36" round affair with two chairs?
I bought a couch, love seat and new (to us) entertainment center from a friend who was moving for a hundred bucks, and got some cheap patio furniture. We were jammed in like sardines, but everyone had a good time.
Last Sunday, Mother’s Day, I got phone calls from: my oldest daughter, my youngest step-daughter (she said she called me, not her dad, and refused to talk to him on that call), and my step-step-daughter (hubby’s stepdaughter from #2-he raised her and gave her away at her wedding last year while her natural father sat and watched) that I usually call my step-daughter once removed. The Sausage Creature sent a card and stuff from Penzey’s
! My oldest step-daughter had sent a package a couple days earlier that contained a margarita/smoothie maker and two Sudoku books. And she knows her dad doesn’t drink tequila! Gee, honey, maybe these Sudoku books are yours?
What am I trying to say in all this? Keep a sense of humor, be yourself, and make their dad happy. Hopefully, the rest will fall into place.
Good luck with your new role!