Abigail (Half Sister of David) gave birth to Amasa (Chief of Joab) with an Ishmaelite named Jether (also Ithra).

We only know that Scripture mentions Zeruiah and Abigail as sisters of David and daughters of Nahash (instead of Jesse). Various explanations have been offered.

1. The daughters of Nahash could have been adopted and raised by Jesse.
2. Nahash could be another name for Jesse.

Since we don't have any other links from Nahash to Jesse, we followed the 1st option and made Zeruiah and Abigail half-sisters of David.