Gemara:


Ein Ayah: The situation of a family when it is in a state of tranquility is an accurate sign of the nation’s completeness in ethics and good personal characteristics. Therefore, it is a wonderful to see the completeness of our forefathers’ holy attributes through the following observation.
It is normal that if a woman is not happy in her husband’s house, her sister will not want to marry her sister’s brother-in-law. When parents see their daughter happy, they and their other daughter will be interested in pursuing another match between the families. This is a high level to reach, and it is especially noteworthy that the families involved were kohanim, as they naturally have strong tempers (see Kiddushin 70b, based on Hoshea 4:4). Despite that, eighty couples of kohanim lived in such bliss that their siblings wanted to marry from the same family.
The foundation of familial peace within the nation is a life of modesty. Therefore, the greater the fear of Hashem and holy purity that exist within the nation, the greater the bliss that will exist within families. It is thus appropriate that the name of the city was Gufaniot, which reminds us of the Rabbis statements that grapes can only accept grafting from other grapes and that "the grapes of the vine and the grapes of the vine" (references to the marriage of children from two good families) is a proper matter (Pesachim 49a).
The interconnectedness of families is also a good sign of the physical and spiritual health with which Hashem blessed His nation when they dwell in completeness in the Land. This is because we know scientifically that when there is health within a family, children of two parents from the same family (if they are not so close that there is incest) are likely to be particularly healthy. On the other hand, when the family has illnesses within it, then the chances of problems with the children are much greater. When kohanim are careful to marry specifically from a family of kohanim, there is usually marriage within an extended family. We see that in the cases in question, the children turned out nicely, or else they would not have continued the practice so often. This shows that there was a completeness of the body and the spirit, which comes from following the Torah, which is a tree of life for those who cling to it.
This ideal personal and family setup is possible only when people are settled in the special Land that Hashem picked out for His nation according to its spiritual makeup. In the Diaspora, the nation cannot rise to such an ideal state. That is why this phenomenon was not found within the communities of Bavel. In the one case where brothers married sisters, only the sisters were from a kohen family. As mentioned, kohanim by nature have volatile personalities, and therefore only with excellent upbringing can they live harmoniously as couples. The attributes of kohanim are certainly more felt in the males, as they are the ones with the special status with all of its halachic and functional uniqueness. Therefore, those successful marriages involved only female kohanot and not male kohanim. Only in Eretz Yisrael does the sanctity allow for charm, peace, and good fortune, allowing for such tranquil families for the nation as a whole.