Benny,

In the case of Achosa she'Hi Yevimtah... by Eshes Achiv she'Lo Hayah b'Olamo, according to Rebbi Shimon:

the case is that both pairs of wives are sisters, but from two separate famiilies. That is, the wives of Reuven and Shimon are sisters, and the wives of Levi and Yehudah (who will be Tzaros Ervah because their husbands were Meyabem the wives of Reuven and Shimon) are also sisters, but from an entirely different family. The second pair are called "Nochriyos" since they are not sisters of the first pair (i.e. they are not from a set of four sisters). Since the second pair, i.e. the Tzaros, are also sisters, we can say about them as well "Achosa she'Hi Yevimta..."

That is what Rashi means in DH u'Mishum Tzarah.

Hope that clears things up. Be well,

-Mordecai