Dana is Teddy's sister, Dan's wife & Denise's ex-sister-in-law.
She’s first mentioned in "Thanks-Hoarding" where she, Dan and their mother are said to be coming to Teddy's for Thanksgiving. They cancel on the day after their mother misses her flight and reschedule for the next day which comes after the episode ends.
Her silhouette is seen in "The Bleakening" when Teddy tells the Belchers about The Bleaken stealing his and her presents while they were both arguing.