Actually, half-cousins were allowed to get married, but that was the closest relationship possible. I think I remember reading it in Laws and Customs of the Eldar or somewhere else. I'm sure if Maeglin had been an extremely nice guy, and Idril had loved him, Turgon might, (emphasis on "might") have allowed their marriage. But since Idril sensed the darkness in him, and most likely told her father about it, Turgon was most probably slightly careful about Maeglin in general.
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But Melkor also was there, and he came to the house of Fëanor, and there he slew Finwë King of the Noldor before his doors, and spilled the first blood in the Blessed Realm; for Finwë alone had not fled from the horror of the Dark.
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