5 Books Found

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"It was about a girl (tween? teen?) and her younger brother. At the beginning they lived in an urban apartment with their aunt and uncle. I don't remember if their parents were dead or if maybe they had been jailed or they were missing, but I think one or both parents knew something the government didn't want them to know. I have a vague feeling it might have been set in the UK or a fictional place that didn't seem like the U.S."