I often walked around in a towel in my own home, as a teenager, and a step daughter. My step father had two daughters also, and never batted an eyelid. Nor did he act inappropriately in anyway. His daughters often ran downstairs half dressed in a rush to get ready, getting dressed, doing their hair and eating breakfast all at the same time.
All of us had probably opened the door in a towel at some point.
They were not being sexual, they were getting dressed, or simply chilling out while their nail varnish dried.

Now I am the mother of four, two of which are teenage girls, and I hope they are comfortable and secure enough in their own home and own body image to walk around in their undergarments or towel or whatever.
I understand you feel embarrassed but she isn't doing anything wrong,are you relating the instances to something sexual?
I struggle to see the impropriety. I suspect you gf doesn't think this is a real issue either and that your step daughter is picking up that vibe from her.
You chose to become the step parent to a teenage girl. You also chose to co parent with someone who obviously has diffrent set of values. I would consider picking your battles because their will be bigger ones than this.
All the best, I hope a simple sit down is the answer and you manage to find a healthy compromise. Although I think Pfrog had it right.