Thanks to the friendly folks at Tracing the Tribe, I confirmed that Philip's father's name translates as Yochanan Avraham, as shown on his stone at left.
Philip's older brother is Barney H. Markell (1874-1944) and his stone (in a cemetery hundreds of miles away) says the father's name is Elchonon or Alchanon Avraham. Barney was the father of Joseph Markell, who married my great-aunt Mary Mahler.
One younger brother is Samuel Markell (1885-1971), who died--I believe--in Massachusetts. He's not in Find A Grave or the Jewish Online Burial records, so I don't yet know his final resting place.
The other younger brother is Julius Markell (1882-1966), who died in Brooklyn, NY. So far, I don't know where he's buried and can't yet compare his father's name with that of Philip, Barney, and Samuel. The saga continues!