*starts working out LotR family trees, so she can avoid writing that essay for a few more minutes*
So... Imrahil's sister was Finduilas, who married Denethor, had Boromir and Faramir, and then died. This makes Imrahil Faramir's uncle.
Eomer marries Imrahil's daughter. So this means that Eomer marries Faramir's cousin, while Faramir marries Eomer's sister. Double in-laws...
Jeez, and I thought the hobbits had convoluted family trees! The ruling families of men seem to intermarry just as frequently as any hobbit bloodline.