Rhoda's sister
At one time wasn't it Debbie, who got married?
Then later it was Brenda, who didn't marry the McDonald's dude.
Debbie appeared in the Mary Tyler Moore episode when Mary and Rhoda went to New York for Rhoda's "little sister's wedding".
Some details do change when a character is spun off into a new show. Rhoda also mentions a brother on MTM. In another episode a visiting Ida tells Rhoda that she's going on to Chicago to visit Rhoda's sister.