Thank you all.
I get 20d now. What confused me is Imogen's definition of "briefly". Mine almost exclusively boils down to "minus one letter". I know that the second part of the answer is a short word for a family member. It's the abbreviated first part of the answer that confused me.
As for 14a, I knew the twins' names. I know they only differ by one letter. I still can't make sense of the parsing. Sorry guys, one of my "I'm so slow" moments, I suppose. All I can see is the T and Y the twins' names and the answer have in common. . .