WMG / 1600 Penn

Everyone else in the family knows that Marigold is gay.
Becca all but raised her, and besides, Marigold seems very open about her sexuality. The reason Emily doesn't know is because she's new to the family, and Marigold didn't open up to her until the first episode. Skip didn't realize because, well, he's Skip.
http://tvtropes.org/pmwiki/pmwiki.php/WMG/SixteenHundredPenn