Boaz tells Ploni that they two are the only possible redeemers. If Orpah had come with, one of them would have married Boaz and the other would be left single and half the land would be unredeemed.
What would have happened if Orpah came with?
Boaz tells Ploni that they two are the only possible redeemers. If Orpah had come with, one of them would have married Boaz and the other would be left single and half the land would be unredeemed.
What would have happened if Orpah came with?
If the redemption is through Elimelech because he has no living sons (both died childless) then by analogy to the laws of yibum, either one marrying Boaz would have accomplished the redemption. The Rambam in the laws of Yibum points out that if a brother with multiple wives dies childless, then any one of the wives (and any one brother) can perform yibum or chalitzah. In this case, since Boaz is not a brother, then the "goail" is an analogy only and not the actual yibum of the Torah. In this case, it would appear that Boaz could have married both or either (if they were not sisters but only sisters-in-law). In actual fact, Rus could have married anyone (technically) Naomi would have gotten some Elimelech's estate for her kesuvah, the remainder would have been divided to become the brothers' estates. Rus would have gotten some of Machlon's estate for her kesuvah (assuming the psak was that she was originally a valid convert) and the remainder would have been inherited by Elimelech's brother. That is one of the reasons why Boaz and Rus have the term "chesed" applied to them.
If Orpah had accompanied Naomi, the same reasoning would have applied. Since she did not, it showed that she was not a valid convert and she was not entitled to any of her "husband's" estate.
Note that the psak of "moav velo moaviah" (only a female Moabite is accepted as a ger) had been forgotten and needed to be re-established. That is also why Shmuel had to write Megillas Rus and Ploni Almoni was afraid to marry Rus. Only Boaz, the gadol hador, was in a position to enforce this ruling.