I'm Christian, but what I'm about to say is my personal understanding. It seems to me that Christianity, Judiasm and Islam all look at Abraham as a sort of patriarch. I think Jews consider themselves descendants of Abraham and I think many Muslims do, too, but by Ishmael rather than Isaac. I view Abraham as a "patriarch" in the sense of being a "father" or example of trusting God. Abraham seems ro have pleased God by believing Him. Perhaps this explanation addresses your question about different groups/sects and their regard for Abraham.
RFK, Jr has made many very bold statements. Some of those statements (as he says himself) would.have gotten him sued if they were not true.