Cousins are ok. Pretty much everyone else is not.
A man may not marry his:
Mother (also step-mother, former step-mother, mother-in-law, former mother-in-law, adoptive mother or former adoptive mother)
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Daughter (also step-daughter, former step-daughter, daughter-in-law, former daughter-in-law, adoptive daughter or former adoptive daughter)
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Sister (also half-sister)
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Father's mother (grandmother)
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Mother's mother (grandmother)
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Father's father's former wife (step-grandmother)
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Mother's father's former wife (step-grandmother)
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Son's daughter (granddaughter)
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Daughter's daughter (granddaughter)
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Wife's son's daughter (step-granddaughter)
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Wife's daughter's daughter (step-granddaughter)
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Son's son's wife (grandson's wife)
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Daughter's son's wife (grandson's wife)
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Father's sister (aunt)
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Mother's sister (aunt)
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Brother's daughter (niece)
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Sister's daughter (niece)
A woman may not marry her:
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Father (also step-father, former step-father, father-in-law, former father-in-law, adoptive father or former adoptive father)
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Son (also step-son, former step-son, son-in-law, former son-in-law, adoptive son or former adoptive son)
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Brother (also half-brother or step-brother)
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Father's father (grandfather)
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Mother's father (grandfather)
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Mother's mother's former husband (step-grandfather)
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Father's mother's former husband (step-grandfather)
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Son's son (grandson)
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Daughter's son (grandson)
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Husband's daughter's son (step grandson)
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Husband's son's son (step grandson)
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Son's daughter's husband (granddaughter's husband)
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Daughter's daughter's husband (granddaughter's husband)
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Father's brother (uncle)
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Mother's brother (uncle)
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Brother's son (nephew)
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Sister's son (nephew)
In Scotland, a man may not marry his great-grandmother or great-granddaughter and a woman may not marry her great-grandfather or great-grandson.