I wonder that too, I'm not sure why else the oldest sister's design would be so distinct from the younger two. Face shape, nose, hair texture, body type, it's a lot of differences when typically when you design siblings the idea is for them to have a lot of visual overlap.
Actually, just went looking and found a shot with all the siblings, and the older brother also looks very different too, a lot whiter, but the baby brother again looks more Asian, more like Miko. I definitely think the older two are meant to be from another marriage. Don't really expect it to come up in the show though.