Robert F. Kennedy Jr. -- Mary's Suicide was NOT My Fault

Robert F. Kennedy Jr. is ADAMANT the suicide of his wife -- Mary Richardson Kennedy -- was not his fault ... but apparently Mary's siblings aren't buying it.

During Mary's funeral -- held Saturday at St. Patrick's Church in Bedford -- RFK Jr. publicly proclaimed that a friend told him not to blame himself for Mary's death, saying, "I know I did everything I could to help her."

The memorial -- which was boycotted by Mary's 6 siblings -- came just one day after a colossal legal battle erupted between RFK Jr. and Mary's biological family -- each side wanted to claim Mary's body for burial.

RFK Jr. won the battle courtesy of a judge ... which allowed him to retrieve Mary's body from the medical examiner -- just hours before the wake.