There's one major difference between the shows. Six Feet Under was a great show - real quality television. Brothers & Sisters is a fun show to watch, but quality TV it ain't.
Beware of SPOILERS!!!
Brothers & Sisters
Six Feet Under
William has heart attack and falls into pool
Nathaniel Sr. dies in accident
Father dies in pilot episode, leaving grieving widow
William and Holly
Ruth and Hiram
Parent's infidelity revealed after death
Kitty returns from NY
Nate returns from
A child returns home to LA
Fisher & Sons Mortuary
Gay repressed brother
Tommy asks Kevin for his sperm. Kevin refuses at first, but then both he and Justin donate sperm.
David asks Claire for her egg, so he and Keith can have a child. She refuses.
Asking a sibling for help conceiving (gay brother either asker or donor)
Julia Walker goes into labor 29 weeks into pregnancy
Brenda goes into labor two months early
Joe kisses Sarah's half sister Rebecca.
Nate cheats on Brenda with his step sister Maggie.
Rachel Griffiths' character has marital problems. Sisterly infidelity involved.
Husband Joe Whedon
Boyfriend Joe (Season 4)
Rachel Griffiths' character has significant other named Joe
Sarah is Joe's son Gabriel's step mother
Brenda is Nate's daughter Maya's step mother.
Her character is also a step-mother
Isn't it fun to waste time on totally unimportant things once in a while?