I posted this query last year, but no-one ever got it so I figured it was worth trying a repost. By the way, it is *not* by Diana Wynne Jones (the only suggestion I got at the time, but it definitely wasn't her). Also, on reading this over, it's possible the younger brother may have been called Jimmie and not Johnny. (It is of course also possible that I'm wrong on both names. Been a long time.)
Pretty sure the protagonist was called Emma - in any case, I'll call her that. A family move in next door to Emma, and she makes friends with the daughter of the family, whom I believe was called Sandra, short for Cassandra. Emma realises this girl has strange powers - she can walk on water (with difficulty) and has a 'mood window' in her room which makes pictures that she can change using her thoughts. However, it turns out Sandra's powers aren't actually all that strong, which causes her to be a grave disappointment to her family (who have much stronger psychic powers). The shortening of her name is symbolic of this; instead of being called her full, unusual, name, she has it shortened to a rather commonplace everyday name to indicate her lack of appropriate special talent.
However, Sandra's parents realise that Emma's younger brother (Johnny, I think) *does* have potentially powerful psychic powers, so they come up with an evil plot to kidnap him. This involves making some sort of simulacrum of Johnny and connecting the two so that the simulacrum will absorb the real Johnny's memories, thus allowing Sandra's parents to keep the real Johnny and send the simulacrum back to Emma's family to pretend to be Johnny for a short while; the fake Johnny will then become ill and die from a disease that Sandra describes as 'leu-something' and which Emma, who wants to be a doctor, works out is leukaemia. (This is because they can't create a fake good enough to fool the family for any length of time.)
Emma, horrified by this, successfully foils their evil plan (with Sandra's reluctant help) and rescues the real Johnny. This takes place after the real and the fake have been connected to start transferring the real Johnny's memories to the fake. Emma has a moment of horror after getting Johnny home when she realises she may have made a mistake and taken the fake instead, but luckily she figures out a way to check by asking Johnny about something that happened at a point where the memories wouldn't have been swapped... um, have no idea how that worked, but, whatever, it turns out it is the real Johnny. I assume they then live happily ever after but can't remember the ending after that.
Pretty sure the protagonist was called Emma - in any case, I'll call her that. A family move in next door to Emma, and she makes friends with the daughter of the family, whom I believe was called Sandra, short for Cassandra. Emma realises this girl has strange powers - she can walk on water (with difficulty) and has a 'mood window' in her room which makes pictures that she can change using her thoughts. However, it turns out Sandra's powers aren't actually all that strong, which causes her to be a grave disappointment to her family (who have much stronger psychic powers). The shortening of her name is symbolic of this; instead of being called her full, unusual, name, she has it shortened to a rather commonplace everyday name to indicate her lack of appropriate special talent.
However, Sandra's parents realise that Emma's younger brother (Johnny, I think) *does* have potentially powerful psychic powers, so they come up with an evil plot to kidnap him. This involves making some sort of simulacrum of Johnny and connecting the two so that the simulacrum will absorb the real Johnny's memories, thus allowing Sandra's parents to keep the real Johnny and send the simulacrum back to Emma's family to pretend to be Johnny for a short while; the fake Johnny will then become ill and die from a disease that Sandra describes as 'leu-something' and which Emma, who wants to be a doctor, works out is leukaemia. (This is because they can't create a fake good enough to fool the family for any length of time.)
Emma, horrified by this, successfully foils their evil plan (with Sandra's reluctant help) and rescues the real Johnny. This takes place after the real and the fake have been connected to start transferring the real Johnny's memories to the fake. Emma has a moment of horror after getting Johnny home when she realises she may have made a mistake and taken the fake instead, but luckily she figures out a way to check by asking Johnny about something that happened at a point where the memories wouldn't have been swapped... um, have no idea how that worked, but, whatever, it turns out it is the real Johnny. I assume they then live happily ever after but can't remember the ending after that.
