Headcanon; Raylen De Vries
Raylen was worried that his sisters wouldn't want anything to do with him, especially Giorgia. In his mind, they were all siblings, but he knew that the girls had never seen him before, and had no reason to want to be in his care.
And, in truth, Giorgia was resistant at first. But she softened to him when he sang to her in Flemish after she had a bad episode with her heart.
Bianca, just a baby when he came, had no problem accepting him right away. She honestly believes that he's her father because she doesn't remember their mother or her biological father.
Raylen did feel conflicted and tried a few times to tell the girls that he's their brother, not their dad. But they didn't really understand. So he lets them call him Papa, and he really does feel like they're more than sisters. They're his girls, even if they aren't his daughters biologically.