Is movie soundtrack composor Danny Elfman related to actress Jenna Elfman?

Yes, Jenna married Bodhi Elfman, who is Danny Elfman's nephew.

https://en.wikipedia.org/wiki/Bodhi_Elfman

Elfman also wrote the theme for the video game Fable. He is also famous for creating The Simpsons main title theme, and his role as Jack Skellington's singing voice in The Nightmare Before Christmas. He is the Uncle in-law to actress Jenna Elfman.

Jenna met her husband, actor Bodhi Elfman, at a Sprite commercial audition in February 1991. They married in 1995, making director Richard Elfman her father-in-law and noted composer Danny Elfman her uncle-in-law.

Is Danny related to Jenna Elfman? Yes, Jenna is related to Danny through marriage. Jenna is married to Danny's brother's son, Bodhi. That makes Jenna his niece.

http://www.bluntinstrument.org.uk/elfman/faq/faq.html
https://en.wikipedia.org/wiki/Jenna_Elfman
https://en.wikipedia.org/wiki/Danny_Elfman