There is an LDS Ancestral file for a Jacob Harpster, born 1756, died 1832, son of David Harpster (b 1736) and Rosina W. (b abt 1740).Michael Harpster was 27 years old when he immigrated to the US in 1741, so David might have been his son (he would have been too young to appear on the ship passenger list for The Molly.
The Ancestral File just gives Jacob's wife's name as Mary, but there is also an IGI marriage record between a Jacob Harpster and Henny Mary Sarah HERZOG in 1776 in Northcumberland, PA.The children listed for this couple are:
Peter Herbster, b 13 Sept 1778, Union PA
Frederick Herbster, b abt 1781, Union, PA, died 1818
Barbara Harpster, b 8 Mar 1785, Union PA, died 12 Jan 1866, Vigo County, IN
David Herbster, b 19 Jan 1788, Union PA, died 28 Jun 1864
Mary Herbster, b 18 Feb 1790, Union PA, died 3 Jan 1859, Pickaway, OH
Eva Herbster, b abt 1791, Union, PA
Jacob Herbster, b abt 1792, Union, PA, died 1852
John Herbster, b abt 1794, Union PA
George Herbster, b 15 Jun 1795, PA, died 16 Apr 1868, Seneca County, OH
Philip Herbster, b abt 1798, Union PA
Anthony Herbster, b abt 1800, Union PA
Elizabeth Herbster, b abt 1802, Union PA
Hope this helps.