Crossposting my response from a discussion earlier this year in the Kiss & Cry to someone who asked a similar question. In this case it was about why there are so many sibling teams in the U.S. & whether there were any in other countries (I think the resource aspect is obvious. The resources go to teams that win):
Well, of course there are sibling dance teams in other countries. I've seen them from all the major dance power countries: Italy, France, Canada, & Russia, though the Russian teams were younger and the most recent top junior team in Canada split--like @90 percent of the Canadian dance field--in 2016.
But, for what it's worth, I'll take a theoretical stab at why sibling dance teams are successful in the larger scheme of U.S. dance if you'd like:
1. Sibling teams tend to experience more success early in their careers. They tend to train together or with the same coaches from an earlier age, have similar technique, and have shared the same partner from very early on. This is
not true of all sibling dance teams. But it is a pattern. A lot of times, the larger field eventually catches up. But not always.
2. They have successful role models. The Shibs are only the fourth U.S. dance team to ever medal at an Olympic Games. People tend to emulate success. The Parsons just won Junior Worlds. The Greens won two novice titles. (Wheaton has had at least three medal-winning sibling teams. The coaches work with very young athletes, and their teams tend to stay together a long time from a young age).
3. The modern wave of successful U.S. dance teams has been characterized by teams that stick together. The U.S. has only three Olympic medal winning dance teams since O'Conner & Millns in 1976. Belbin & Agosto, Davis & White, and the Shibs all won junior national titles together. I attended the U.S. Championships in 2014. Of the junior teams that split since that competition, none remain competitive. In such a deep senior field, partner swapping seems like a first class ticket down in the standings.
4. Need. The U.S. has a long history of doing a lousy job recruiting young men into the partnership disciplines in figure skating. Brothers are handy. And success encourages them to keep competing.
5. Contrary to the opinions on this board, I see no stigma toward sibling dance teams among the audience members at U.S. Nationals. I suppose it's probably an issue for some people. Everything is an issue for some people. But it would never have occurred to me, prior to reading discussion board comments here, that being a sibling dance team would be viewed as anything other than a plus for the Shibs. They are so well matched . . . it is like they share the same gene pool or something

. As long as I have been watching figure skating (since the 84 Olympics), there has always been a successful international sibling dance or pair team on the scene.
6. The one problem I often see splitting up sibling teams--mostly pairs--is that as they grow up, the childhood height difference often disappears. Maybe this leads people to think that sibling teams can't last. But--you know--precious few young partnerships last. Related or not. You have to be very, very good to last in this sport. I would say that more dance teams succeed today, though, than in the past. We see more movement, and therefore, more successful teams. Including more successful sibling partnerships. And, in dance, the height disparity need not be nearly as large as in pairs. Denkova & Stavisky won two world titles and were quite similar in height. Synchronicity, close skating, good lines, similar technique--all things that tend to characterize sibling dance teams--these can outweigh the advantages of a large height difference, IMO. Not every sibling team has that magical persona that draws you to them. That's for sure. But that's true of the vast majority of non-sibling teams as well.
Finding that kindred-spirit partnership. That can happen at any time. Sometimes it even happens with your very first dance partner.
Additional Note: As far as my own personal observations go, I've always thought the Shibs had that IT relationship with the audience & each other on the ice. I think the Peals have it too.