There are a few dynamics at play here. First, agree that whilst consistency is ideal it’s not realistic with two households and there are far more important things you will want to be on the same page about than where she sleeps. As others have said, mum’s is the primary residence and so long as she is not parenting in a dangerous manner then she effectively
takes the lead.
As you aren’t a parent you may have some unreasonable expectations of parenting (as we all do before we become full time parents) in that it’s also totally normal to ‘start from scratch’ with your kids over and over when working toward something you want them to do, especially if they would rather do something else. That would happen with your own child too chances are.
I have found that being a step-parent the main difference for me is the physical boundary, especially where I didn’t care for DSS from being a baby or change nappies etc. So when he started staying overnight with us when turning 5 I definitely wouldn’t have coslept with him. He would come to us for a nightmare etc and have a cuddle in bed and then one of us would take him back to his room and stay until he slept. However, if he hadn’t been able to sleep independently he would have slept with his dad in our room and me in his.
I cosleep quite a bit with DD who is 2, most nights at the minute she comes in but it is usually around 5/6am. With my DSS, I would cosleep with him now if he really needed me to, but I know with him it would become an expectation, whereas with DD2 she has always gone through phases of cosleeping and then wanting to go back into her own room and always falls asleep in her own room (though I lie next to her until she is asleep). The main difficulty with DSS is that he is a noisy and fidgety sleeper and I am a light sleeper so wouldn’t get any sleep, and I know he is a heavy sleeper so once asleep will always sleep through in his own room without waking. So he has a nightlight and I rub his head to help him sleep if he needs it. The fact we have never coslept also plays a role as it’s probably the only boundary in our relationship that is different from with my biological child, so it would feel a bit odd to suddenly do it now that he’s older, but again, if he wasn’t going to sleep otherwise I’d certainly consider it.
DD2 is nowhere near as disruptive in her sleep and the quality of sleep for all parties is really important in terms of sustainability and impacting your relationships overall.
I think so long as your DSD is happy that she can come to you when she needs something, day or night, it’s totally fine that she sleeps independently at yours and not at her mums. Also totally normally for them to ask you to get them a drink etc in the nighttime, I would expect this until they are secondary age at least, not least because I’d otherwise panic at being woken to a noise in the house that wasn’t a child coming in!