Maybe the extremity of the behavior of the two Swedish sisters who threw themselves into traffic on the M6 was transient, but since drugs were not part of the equation, it seems much more likely that the behavior was connected to something more hidden (from our view at least) and more long lasting. The "Folie à deux" (induced delusional disorder) is plausible, but was this just transient madness without drugs?
What about the family backgrounds of these two women? They certainly chose to go far, far away from home as adults. Why did they choose to do that? Just adventurers or did they feel they had to escape something (including each other). Similar experiences growing up as twins would have primed them even more for Folie à deux (shared psychotic disorder). After all, even when it isn't psychotic, family members often sway each other mentally.
What about the contents of their bags? It seems that the paranoia there was a bit elaborate. Perhaps their behavior could have peaked into some sort of transient madness, but it was probably just because the madman (or mad women) woke up, not because it was manufactured instantaneously from nothing. The level of disconnection / over-connection with Ursula that Sabina displays seems like two sides of the same coin. She is simply not grounded in reality on some level. She needs both to be nice and ordered and also to deal with intimacy issues at the same time. Probably, this grounding was not established when she and Ursula were small children. Perhaps they developed massive systems of denial that in many ways worked for them (as worked for Sabina in the jail scenes).
After she's released from custody, Sabina greets total strangers on the street - bold and unfearing and then goes out with them. Maybe it her small way of saying "I can be risque without really getting in trouble, ergo, I am in control even in situations that might be out of control." Just a thought. She offers them cigarettes and then snatches them away. "They are poisoned." Is this just a random paranoid thought? Or a symbol of extreme inner anxiety that she suddenly can neither access directly nor ignore? Maybe she is torn between doing favors for people in order to get positive attention and the fear that they will poison her. It just seems that it was her way of retracting an offer that she suddenly felt very uncomfortable with. She is too friendly on one level, paranoid and guarded on another level.
She really wants help, but believes that the knife that cuts the sandwich for her is going to cut her as well. Maybe that's why she thinks that she has to stab a stranger in the chest. There is nearly always more to the story, especially in cases of mental issues. The mind is a black box of history and mystery.