Boy (and sister?) adopted by family in Montana or thereabouts. They go to the local (small) high school, discover ley lines on the property (in a cave as I remember). Extradimensional travel and lengthy stay in a technologically backward dimension, then they return to battle some sort of mind mage who controls numerous other mages under color of some federal bureaucracy. Two girls from the other dimension, now part of the protagonist's harem, and who are not affected by magic, help to save the day.
Been looking for this one for a while now. Any help will be appreciated!
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