‘ And Gilead's wife bore him sons, and when his wife's sons grew up they drove Jephthah out, and said to him, “You shall not inherit in our father's house. For you are the son of another woman.'
It would seem that Jephthah was Gilead's first child, whom he took into his house. But then his own wife bore him children, and as they grew up the question of inheritance cropped up. One problem was that he was the firstborn, (although not legally), and assertive. We can understand why they feared for the future. But even the child of a prostitute could expect some kind of inheritance from his father when he was a part of the household (compare Genesis 25:6), and he certainly had a right to his father's reasonable provision. They, however, begrudged him even that, which was why they drove him out. As Gilead would presumably not have permitted this we must presume that he was either ill, or more probably dying, although it may be that he was driven to it by a constantly nagging wife, as Abraham partly was by Sarah (Genesis 21:10-11).
Yet as a bastard Jephthah and all his descendants would be barred from entering the assembly of Yahweh, that is from becoming full Israelites, for ten (or ‘a number of') generations (Deuteronomy 23:2). It took that long for the taint to be removed. His position was an unhappy one. Interestingly the same was true for their foe, the Ammonites (Deuteronomy 23:3), or even worse, because their barring was ‘for ever'.