Is it morally legal for a father, brother, and son to be interested in marriage with three unrelated females that just so happen to be a mother, sister, and daughter, related to each other, but not to the men? If not, which ones and why not?
Though we are not under the Old Law, this is the only law that has details similar to your question. In Numbers 18:6, a person was forbidden to have sexual relations with a close blood relative. This was spelled out in detail in Numbers 18:7-18. Since the women in your scenario are unrelated to the men, this law does not apply.
Nor do I know of a civil law that would forbid such unions.