The parents of sextuplets who were born last week in Montgomery County have gone public. They are Stacey and Brendan Carey of Feasterville in Bucks County.   A staff of about 60 worked on their delivery early last Wednesday morning at Abington Memorial Hospital.  Three boys named John, Patrick and Connor and three girls named Emma, Samantha and Olivia weighed between one pound, one ounce and two pounds, five ounces at birth.  Doctors say a term baby is born at 37 weeks, the sextuplets were born at 27 1/2 weeks, and they remain in critical condition with complications from being born prematurely.  They will probably stay in the hospital for ten to twelve weeks.