I mean yeah Non Blood Related Siblings, cool, whatever. What's fucking wrong with the parents though????? Like the kids just got off of vacation and they are about to go to school and you drop the bomb that you are divorcing. While they are still reeling from the implications that the family life they have always known would be different but at least they have each other then you drop another bomb about how everything they have thought up until this point is a lie and that you will in fact be separating them from each other so they don't have any family anymore? And that's just an early morning conversation for you to drop on them? That's kinds fucked up.
Both of them has a parent they've never heard a peep about. There's also grandparents and other extended family members, while dealing with the tricky issue of how much of a relationship you'll have with your now ex-stepparent and their family, and it's not looking too good, with the "mother" taking the sis and moving far away.
They could very well have relatives their age they never got to bond with, or grandparents that would have built a relationship with them, but nope, they're young so let's just move together and pretend those people don't exist, 'cause it's not like it's going to bite us in the ass some day.
The low-empathy approach the parents took kind of explains why the siblings got so intertwined - to replace the parental love and affection that was lacking.