credit: duderocks
It is about time we had a thread for all the people on ER who shaped our favourite characters, for better or worse, so please feel free to discuss any sibling, parent, or person related in any way to the main cast. Also use this thread to post any news you find about what the actors are up to.
Don't forget that we already have a thread devoted to the kids of ER
http://www.fanforum.com/f54/er-kids-...them-62946294/ 
Some of the more notable actors who portrayed family members:
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Sally Field as "Maggie Wyzenski", Abby Lockhart's mother who suffered from bi-polar disorder. Sally played the character on/off for 6 seasons and won an Emmy Award for Outstanding Guest Actress in a Drama Series in 2001 for her role on ER.
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Frances Fisher as "Helen Kingsley", Kerry Weaver's long-lost mother who conflicts with Kerry over sexuality and spiritual beliefs.
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Frances Sternhagen as "Millicent Carter", John Carter's grandmother who ran The Carter Family Foundation. Frances played the character for almost 6 seasons on a recurring basis and her storylines involved funding Carol Hathaway's clinic, battling with Carter over how to treat Chase's drug addiction, and attempting to help Carter after his stabbing.
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Kathleen Wilhoite as "Chloe Lewis", Susan Lewis's wayward sister who struggled with alcohol and drug abuse. Kathleen appeared frequently in the first two seasons of ER and returned for a guest appearance in Season 8.
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Khandi Alexander as "Jackie Robbins", Peter Benton's sister who often urged Peter to realise the importance of his family over his career. Khandi made sporadic appearances during the first 8 seasons of ER.
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Judy Parfitt as "Isabelle Corday", Elizabeth Corday's mother who had a brief relationship with Mark Greene's father. Judy appeared in 7 episode of ER from 2000-2002.
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James Farentino as "Ray Ross", Doug Ross' long-absent father who was ultimately killed in a car accident in Season 4.
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Sam Jones III as "Chaz Pratt", Greg Pratt's younger half-brother who trained as a paramedic. Sam had a recurring role on ER from 2005-2009.
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As decided by us and as showing by the thread banner, the ER board's favourite family members are as follows:
Best Parental Figure: Maggie Wyzenski (played by Sally Field)
Best Sibling: Eric Wyzenski (played by Tom Everett Scott)
Best Spouse: Sandy Lopez (played by Lisa Vidal)
Best Child: Reese Benton (played by Matthew Watkins).