I go with what it says - Mother, Sister, Fiancée or Wife (and I'd guess partner now).
The only one of the above that has worn mine was my sister - she thought it'd look good next to her Gamma Phi badge and presidents "dangle" thing... or as she called it her "bling"...
She has worn it on a few of her road trips down south, and to her conference - was only challenged on it by an AM... who promptly lost his pin somehow and didn't get it back until he offered many liquid 'apologies'