I have had a very hard time understanding why DH would be perfectly ok with FauxSGD coming out on top but if it was ever either one of my kids, it was unfair.
A couple of examples of what has happened
We all go out to dinner YSD would come with us, she is DHs actual bio. There would only be enough appetizers for everyone to have a set amount and then one extra. With my kids they are told, sorry not enough. Then DH would go to give it to SGD. He would get upset if. I would say, hey there is not enough for everyone to have more so that means SGD. His response would always be well, it will go to waste. I would respond with DD and BS just asked and were told no. So NO fauxSGD can't have it. Then he would pout along with her or give her bread. OR the times that would piss me off, he would give it to her anyways. This girl is already over weight.
Rule.. Eat all your food or no dessert. DH but it's not fair DD got dessert. Well, it's not fair DD ate all her food and followed the rule but you want fauxSGD to get the same without doing the same

Or weekend chores. It not fair DD and BS are at their dads (DH wasn't letting fauxSGD go to fauxOSD) and SGD has to clean her room. You need to have them clean during the week (we call it Sunday clean, it is the deep cleaning of their rooms). Visitation started up with OSD, SGD went a month without taking the trash out of her room, 2 weeks without doing laundry .. But it was OK that DD and BS did it during the week.

Do your homework and chores... Get electronic time. SGD who lived with us for 3 plus years had a chore for 2 months. DH fired her because he got tired of dealing with it. Same reward as DD and BS without the same work. Because it's not fair.

My kids have a weekly and daily chore list on the wall in their room. They are in page protectors (so DD/BS can use white board markers and cross them off) SGD would do her homework and be on electronics double the time DD and BS got for finishing their homework, personal (their pets or rooms or bathrooms) and household chore.

Good forbid it ever be brought up. DH would go on about well, they aren't that responsible because you remind them or you don't let them on electronics until it's done so it's not like they remember. He would argue with me that them doing their chore doesn't count if I remind them so it is the same as SGD not doing one.

Seriously, were does he come up with this stuff?? This list would be a mile long if I listed it all. But then he wonders why I don't like being around her. If I or he corrects her in front of anyone it will embarrass her, so he would go in her room behind close doors but no problem telling my BS to behave in front of everyone.

Since she moved back in with fauxOSD the issues in the house are almost all gone. We can parent DD and BS together. We can run our house great without SGD but every once in a while I get a flash of resent when he tells my kids to do something when he would have NEVER had SGd do it. The flashes are getting less the longer she is gone, but I know if she ever moves back, I'm moving out. We were headed for divorce before she left.