In Vayigash (Bereishis 46), we are told that Yaakov went down to Egypt with seventy souls, and they are enumerated. The count equals 69, and there are various answers, such as that we are counting Yosef, who also came to Egypt earlier and came from Yaakov's loins; or that we are counting Yaakov himself; or that we are counting Hashem; or that Yocheved was born between the walls; or that 69 is essentially the same as 70, and that is the way Scripture speaks. There is also the irregularity that we have only Dinah bat Yaakov and Serach bat Asher listed for the women; is it really plausible that these were the only daughters, especially when elsewhere (Bereishis 37:35) we have reference to Yaakov's daughters, in the plural?
We can answer, perhaps, by considering the purpose of mentioning the seventy souls. In Torah, it serves two purposes. First, to show how far Yaakov has become. They start out as a small family, in the time of Avraham and Yitzchak, and now, when coming down to Egypt, they are an entire clan. That is the point in Vayigash as well as in parshas Shemos (perek 1) when, again, the figure of seventy souls is mentioned. The second purpose, to provide contrast of small clan to a nation. In Devarim 10:22, Moshe says that with (merely) seventy souls you have come down to Egypt and now Hashem has made you multitudinous like the stars of the heavens.
If we consider this seventy souls canonical and the main point, then the specifics of who the seventy souls are is not so important. And it can be idiomatic, or even excluding all the daughters and sisters. But in the thread of the Torah in which genealogy is important (call it P if you want), especially to set the stage for the next stage of Jewish history, then we want to spell out those seventy. And so, by pulling from genealogical lists found elsewhere in Torah, the Author puts together a list of mentioned personalities who can make up those 70. And we know of Dinah because of the incident with Shechem, and we know of Serach from Bemidbar 26. We *apparently* don't want to include Er and Onan, because even though they came from Yaakov's loins, they did not descend to Egypt. But maybe they should be included instead of the two exceptional daughters.