I'm preparing to teach the book of Ruth for the first time. In fact, to even study it! I'm a bit confused as to why Naomi insisted that her daughter-in-laws not go with her to her homeland in Judah when Moab was a land full of idolatry. If her daughter-in-laws married men there, would they not be men that were not believers in the true God?
Also, wasn't it "Naomi" that when she saw how determined that "Ruth" was that she didn't say another word.? (1:18)
Were Naomi's daughter-in-laws sisters or sister-in-laws?
While you are looking at it from a spiritual standpoint, I believe Naomi was at the time looking at matters from a financial viewpoint. Her husband had died along with her two sons. The reason she was returning is because there was food available there (Ruth 1:6). She told her daughters-in-law to return because she had nothing to offer them. She couldn't offer them husbands, such as under the old custom of a widow being married to her dead husband's brother, as she had no surviving children, she was too old to have children, and even if she could have more, it would be too long of a wait for two young women. It is not that she didn't love them or that they didn't love her, it was the simple fact that she had nothing. One sorrowfully left knowing that what Naomi said was true. The other, Ruth, didn't care that Naomi couldn't offer her anything; she was going anyway.
Yes, if her daughters-in-law remained in Moab, they might not marry godly men as they had before. But they might not find any husband at all. But they would at least have their families there to support them. Still, Naomi couldn't offer them prospective husbands in Israel and she would not be able to support them while they remained unmarried.
Yes, when Naomi saw that Ruth could not be dissuaded she stopped trying to talk her out of coming to Israel with her.
There is nothing in Ruth to indicate that Orpah and Ruth were sisters, so there is no reason to conclude such a relationship.