I have a rush tee shirt with my fiance's letters that I wear - they gave it to me, it's obviously something the culture of their org is okay with. And it's comfy and has pirates on it. But he understands he's never going to be wearing my letters
(Well, I guess maybe for OPhiA - I don't know if the policy on that has quite been settled on that ... it's been heated.)
Regarding parents and letters - my little brother brought home a bumper sticker of his letters for my parents to put on their car that the fraternity had given out for that purpose. I thought that was really odd (so did they) - I mean, why would your parents have your letters (unless they happen to be an alum/na)?