As we know, all our foremothers came from homes of [famous] idolaters - Betuel and Lavan (well, Sarah also).
Once Avraham started the dynasty with Itzhak it seems reasonable that all newcomers will pass some [even short] ceremony of declaring their adherence to monotheism and observing its Mitzvos. In fact, we see that no such ceremony for Rivka, Rachel or Leah or else.
If they didn't, how could our forefathers marry them and what makes their children Jewish? If they did, why does Torah omit such an important detail?