Oh crap.
She's his actual Oneesan.
I don’t think so. Their hair is similar in color, but not exactly the same. Characters having the same hair/eyes is the universally understood way of showing they’re related. It wouldn’t have impacted their designs much to make them the exact same color and yet the author chose not to. I don’t think we’ve ever seen Yuuta’s eyes so we don’t know if they’re red like Luca’s.
edit: I checked the earlier chapters and Yuuta’s surname is yokomine, which is different from Luca’s. She could have changed it, but so far there has been no evidence that she is his blood related older sister. I have noticed that the ends of her hair have a more bluish shade that is similar to Yuuta’s, but it’s still difficult to point to that as evidence of relation.