Question: Who was Amasa's father? In 2 Sam. 17:25 it says that Amasa's father was Ithra an Israelite. But in 1 Chr. 2:17, it says that Amasa's father was Jether an Ishmaelite. This translation is the same in the NIV, is this not a contradiction?
Response: No, there's no contradiction. A footnote in the Concordia Self-Study Bible NIV says that "Ithra" is a variant of "Jether." In Hebrew, the first three letters of "Ithra" and "Jether" are the same. "Ithra," however, has an additional, fourth letter at the end. It is similar to variant names that we have in the English language, such as "Joe" or "Joey."
As for whether he was an Israelite or a member of another tribe - perhaps he was both. Many people, throughout the ages, would be members or more than one tribe, or clan, or group of people, depending on the heritage of their parents. A person, for example, might have a French father and a German mother, and would therefore be both German and French.