What's the Name of That Book??? discussion

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The Obtuse Experiment
SOLVED: Children's/YA
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SOLVED. YA Sci-Fi. Group of misfit teens win or are awarded a free cruise ship holiday out of England? They come across an alien (appears human) who helps them with something. Read in 1990s. Spoilers ahead. [s]
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Natasha, where is the cruise ship headed? Who seems to be offering the prize?
I added a few details to the header/ topic title. Feel free to edit it.
I added a few details to the header/ topic title. Feel free to edit it.

The cruise ship was headed around northern Europe I think, but it was definitely unclear who was offering the prize - it was some sort of government award for kids who had experienced tough stuff.
GENRE: YA fiction/sci-fi
PLOT: A group of older teens, mostly (or all) from England I believe, are chosen somehow to go on this cruise and an alien of some sort joins them, but he appears human and they don't discover he's not human straight away. At one point he gets really sick, but they don't want to let on that there's a stranger on the ship, so the protagonists ring their parents who are doctors (I think), and they get really mad because they're describing an illness commonly found in dogs, not humans.
CHARACTERS: All of the characters are misfits somehow. One I remember killed his mother by smothering her with a pillow, but he did it because she was suffering from advanced MND and he wanted her not to suffer any more because he could see she was in terrible pain and couldn't watch it anymore. There are also twin girls who have different birth dates because one was born just before, and the other just after midnight.
SPOILER: (view spoiler)[As the story unfolds it is revealed that the alien is there to stop the cruise ship being blown up by the government because they're all those kids that the government doesn't want let loose on society because they push back and question the status quo. (hide spoiler)]
THANKS: In advance for everyone on this thread! This is amazing!