The marriage laws in Massachusetts specify the following:

No man shall marry his mother, grandmother, daughter, granddaughter, sister, stepmother, grandfather's wife, grandson's wife, wife's mother, wife's grandmother, wife's daughter, wife's granddaughter, brother's daughter, sister's daughter, father's sister or mother's sister.

No woman shall marry her father, grandfather, son, grandson, brother, stepfather, grandmother's husband, daughter's husband, granddaughter's husband, husband's grandfather, husband's son, husband's grandson, brother's son, sister's son, father's brother or mother's brother.

Presumably the demand for such marriages is small.  But should policy prohibit them?

Thanks to Joel Pollak for the tip (he noticed a sign with this information when he applied for his own marriage license).

Pollak is the Republican (libertarian sympathizing) candidate for the 11th Congressional district in Illinois; see here for more information.  I met Pollak recently when he visited Harvard.

See here for a faceoff over subprime lending between Pollak and Barney Frank; see here for a follow-up interview of Pollak by Greta Van Susteren.