Ithra an Israelite = Jether an Ishmeelite. See 1Ch_2:17.
went in to. Probably meaning seduced. If so, it would be during David's stay in Moab (1Sa_22:3, 1Sa_22:4 Abigail. David was probably half-brother to Abigail, and Zeruiah, having the same mother; he having Jess for his father, they having Nahash.
sister to Zeruiah. Implying that she was not sister to David.