Joint family is best as far as family experience goes. For example, shared parenting, shared responsibility of kids and other issues in family. And there are many other small good and bad experience that benefit. Only issue that may arise in modern families is lack of space, privacy, and ego issues. If those are taken care of then I assume that a lot of families benefit from joint family instead of nuclear one.
Nuclear families these days are facing a lot of issues. Some of the issues include kids being raised in daycare ending up in more psychological issues. And also they not caring for the people around with lack of empathy as they grow up. Also nuclear families end up isolating themselves on many points compared to the joint familes. Then there are some of the benefits too which we can't ignore.
It all comes down to how much understanding two brothers have. How good their spouse handle each other. How the arguments and the issues are solved in the family. That should decide the direction of the joint family or nuclear family decision.