Step kids (kind of 🤷🏼‍♀️) what does this mean⁉️

So what does it mean when a guy you’re newly dating but have known each other for about 10 years starts to bring his daughter (from a previous relationship) around you. And not like briefly but sleepovers and I even have her when he’s working, so it’s just me and her while he works an entire shift. She’s 9 so she’s a big girl but after all these years I’m just now actually meeting and getting to know her. So for all the co parenters out there is this just something that’s acceptable? Moms cool with her being around me, dad obviously is too. What’s this mean? I not completely comfortable with the step mom title just yet because me and her dad aren’t all that serious more so of a fwb type of relationship. Well that’s how I view the relationship. I don’t want to ask her directly like is this the norm for her but I’m so curious and I don’t want to bring it up to him because I don’t want him to feel like he’s made a bad decision because I actually look at her like she’s a part of the family and treat her no different.