Geezer Butler is mourning the death of his older sister.

The 70-year-old BLACK SABBATH bassist revealed his sibling's passing in a social media post earlier today. He posted a childhood photo of the two of them, along with the following caption: "Farewell my dear sister Eileen. Rest in peace."

A founding member of BLACK SABBATH, Butler is also the lyricist of such SABBATH classics as "War Pigs", "Iron Man", "Paranoid" and others.

Butler, singer Ozzy Osbourne and guitarist Tony Iommi reunited in late 2011 and released a comeback album, "13", in June 2013.

In February 2017, SABBATH finished "The End" tour in Birmingham, closing out the quartet's groundbreaking 49-year career.

"The End" was SABBATH's last tour because Iommi, who was diagnosed with cancer in 2012 and is currently in remission, can no longer travel for extended amounts of time.

The original lineup of SABBATH came together in 1969 with Iommi, Osbourne, Butler and drummer Bill Ward. That lineup recorded and toured through 1978, and periodically reformed through the '90s and 2000s for live work.

They regrouped again in late 2011 for a new album and tour, although Ward dropped out after a few months over financial issues. SABBATH has used Ozzy's regular touring drummer Tommy Clufetos since then for live work. RAGE AGAINST THE MACHINE's Brad Wilk laid down the drum tracks on "13".

