I was hesitant to reply because our situation, while in some ways similar, is also quite different. I have a very strong bond with my step-daughter, and she is absolutely part of our family. In fact, we planned our last pregnancy so that she would be with us when her littlest brother was born, so obviously I don't have any of the same concerns about having her around after birth! She was with me while I was in labor with her oldest younger brother, went to the neighbor's for a couple hours, then returned an hour or two after he was born. She was with her mom when her next sibling was born (has she been with us, she would have gone with her brother to a friend's during the labor and delivery, then returned as soon as practical afterward), but my husband picked her up the next day as per our usual schedule. And, that said, even though she is sweet and helpful and loving toward her siblings, I'd just had a baby and relied on my husband to keep the older kids (including my step-daughter) entertained and the house somewhat quiet and peaceful for those first few days. He took them on lots of outings, walks, etc the first couple days.
The circumstances of our husband's non-relationship with the mother of their oldest IS something we have in common, as is our decision to move away in order to better meet the needs of ALL the children (including my step-daughter). I'm not going to get into a discussion of whether or not other people believe our choice was the right one, but I understand what it is to decide to move away and to believe it is better for the health of the family overall even when it is in some ways harder for individual members of that family. So, at the very least, don't feel like you are alone in thinking that it isn't always selfish to move away, and it isn't necessarily a case of sacrificing your step-son's happiness for the sake of the other people in the family. That has not been the case for us at all (and we moved much further away).
There are two things that stand out to me as really positive about this arrangement for my step-daughter. First of all, my husband travels to her mom's state every month and spends 4-11 days JUST WITH HER. While he has occasionally brought one of the other kids with him, and once a year we go as a whole family, this is time that is dedicated just to the two of them. I think it has made their relationship really strong, and they have an amazing set of memories together of some incredible father-daughter time that she wouldn't otherwise get. And it allows him to have a turn to help with homework, volunteer in the classroom, take her to lessons, meet her friends and friends' parents, which is important for that real parenting bond.
The other thing that has been a great benefit is the long stretches of time we get to spend together without interruption. We used to have 50/50 custody and she went back and forth every couple days. That arrangement definitely had its benefits, but we rarely got long stretches of time to be together... there's something about having aweek, two week, several weeks that really bonds a family together. And she is with us when we are on vacation, so we really get to focus on being together without having to worry so much about work and school and schedules. I think having those long stretches has helped keep our family bond strong in between visits. It allows her to settle in (all of us to settle in) and really feel like she lives here. She has friends here who she looks forward to seeing, and we are able to enroll her in extracurricular activities so she makes new friends through those as well.
It is absolutely challenging... it takes a lot of time, money, and dedication. If you are lacking in any one of those things, I think it could be disasterous. We dedicate a huge amount of energy to making sure we are doing the absolute best we can with the less-than-ideal circumstances, and it is no small feat. I'm not sure I could do it if I weren't 110% committed to her, and if I didn't absolutely see her as one of MY children, worth the sacrifices and effort that I give to all af them. It is hard on all of us, all in our own ways and to varying degrees... not in the same ways that it was hard on us to live where we were, but in different ways that can be just as challenging, sometiems moreso. To do it well, it is HARD work.