I would think that you used "sono" in reference to the people inside the car. Albeit, the sentence may be a little confusing because the subject is "La macchina" in the first part of the sentence, but in the second part the subject is a dropped "Essi". You also are missing a conjunction to link the two phrases. The expression "vedersela brutta" is also a bit...dramatic for the sentence. I would only use it in the instance of a really horrible car accident, in which I had almost died (such as the one that occurred in Avellino recently).

La macchina ha cominciato a sbandare, se la sono vista brutta....è un'espressione usata per dire che chi era dentro la macchina ha preso un BRUTTO SPAVENTO oppure SCAMPATO PERICOLO.....