All through history, you can find places where the Jews split and some became apostate. Ishmael has been brought up several times and he is a prime example. I agree that the definitive line for the Jews would be Jacob, but I do think it is interesting about the blessing to Japheth that he would dwell in the tents of Shem.
I would not say, though, that Ishmael is the father of the gentiles. He was the son of an Egyptian woman probably of the line of Canaan (IMO). The gospel plan was to bring the Gentiles into grace also, but it would not seem that Ishmael fits this category, "He shall be a wild man; his hand shall be against every man, and every man's hand against him. . ."
I do not think it to be a really important argument and I can see others' side also, but I still question the curse and blessings that I mentioned above and to me anyway, it would seem that you have the representation of three groups of people. Jews, Gentiles, reprobate.