Who is David’s mother?

 ²⁵ Absalom set Amasa over the army in place of Joab. Now Amasa was the son of a man whose name was Ithra the Israelite, who went in to Abigail the daughter of Nahash, sister of Zeruiah, Joab’s mother. (2 Samuel 17:25)

And,

¹³ and Jesse became the father of Eliab his firstborn, then Abinadab the second, Shimea the third, ¹⁴ Nethanel the fourth, Raddai the fifth, ¹⁵ Ozem the sixth, David the seventh; ¹⁶ and their sisters were Zeruiah and Abigail. And the three sons of Zeruiah were Abshai, Joab and Asahel. ¹⁷ Abigail bore Amasa, and the father of Amasa was Jether the Ishmaelite. (1 Chronicles 2:13-17)

I believe if Zeruiah and Abigail were their sisters, it would mean all of them, Eliab through David. Therefore, if Zeruiah and Abigail’s mother was Nahash, so were theirs. If there was unfaithfulness or anything else, it would have been stated somewhere such as Amasa’s paternity.

References

2 Samuel 17:25

1 Chronicles 2:13-17