I think there was a girl who discovered some telekinetic powers in some kind of car accident. An old neighbor lady turned out to be her helper or guardian and told her she was an alien. The girl's brother (maybe twin) is disappointed he is not. The girl had faked some blood tests in school because she kind of knew she was different, maybe because of the color of her blood? I read this as a kid, probably around 1986, translated into Danish. Sadly, I can't really remember it.
This sounds a lot like Gilead, one of the stories in Zenna Henderson's "People" series about human-like aliens who crash on Earth and have to fit in with normal folk as best they can.
The sister Bethie, is a sensitive who can feel the pain of those around her. Her mother helped until she died. The two left to try and find others, and have a car accident with others of the people who are levitating their car. The brother does have telekinetic powers, but he is disappointed when he thinks they will be rejected due to their half-blood status. In the end, they helped Bethie control her gift, and they join the People.
From a character list of the People stories:
Central figure and narrator of "Gilead" (pg. 50-79). Son of one of the People who was separated from the others, and who married a human, Peter is only half-People, but is talented at levitation. [Last name first mentioned on page 51.] Appears at end of "Wilderness" on pg. 161. Mentioned in "Jordan" on page 234 and elsewhere.
Younger sister of Peter, and the other main character in "Gilead." Only half of the People, she is a Sensitive, and has the ability to feel the physical pain of others, but lacked the ability to control that pain while growing up. Also appears in "Pottage," beginning on page 109. Appears at end of "Wilderness" on pg. 161. Mentioned in "Captivity" on page 214. Mentioned in "Jordan" on page 234 and elsewhere.*
This sounds like The Girl Who Knew by Sandra Glover.
From the Amazon Store page:
I believe this book may be the novelization of A Distant Home, a 1992 Australian TV movie. A great many elements fit: the girl is an alien but her twin brother is not; her alienness is revealed by a car accident; a neighbor lady turns out to be a guardian from her home planet. Perhaps you can check this description of the books at Goodreads and see if it fits.