When your SKid/kids go off with BM and come home with stuff, do you feel the need to go out and get things for your bio-kids to make it "fair"?
SD often comes home with stuff from visits with BM, or stories of the things BM does with her (Build-a-Bear workshop, movies, ice cream treats, fast food, etc). My bio-kids have never made a big deal out of it, if they have a problem with it, I just say, "SD must have had a really great time with her mom, huh? It's nice to be able to spend time with the people we love isn't it?"
The only time any of the kids have ever complained was when DS started school and realized that his school had SD's grade in it. He was SO excited because he thought that meant sister could just go to his school and stay at his house forever (this was when BM had custody). We had to explain to DS that it didn't mean sister could stay forever and go to school with him.
Also, it seems unfair to SD for me to go out and get the other kids something and leave her out. It's not like her BM is taking all 4 kids (SD and my 3 bios) and only getting SD something while they sit back and watch.
I've always thought that fair means everyone gets what they need, not everyone gets the same thing.