S.B.
YOu're the stepmom and Kallie is coming to see her dad - he has to make the time and effort to be there for her. He has to take the lead in introducing her to the family and in integrating her into the family. She's not related to you in any way and may feel uncomfortable if you take too strong a role in acting the mom. You need to step back and let her indicate how comfortable she is with you. Don't assume she is going to want a close relationship with you. Let her lead.
I agree with others who have suggested making sure she has privacy and a comfortable space of her own to retreat to. Let her choose how much time she wants to spend with the younger children. Have meals all together and arrange some age appropriate activities for the whole family, including her dad. You don't know her interests yet, or what kind of kid she is, Take it slow, and let her show you what she needs.
take care, S.