I think there are certain things to look for. Are the skaters able to take instructions? Are they able to cooperate together as a team, or does either of them prioritise constantly himself/herself... .
That's such a great answer! When it comes right down to it, I think that such aspects of character and personality that make cooperation a priority are much more important in a successful pairs skater than physical skills, which can be taught. Even physical bodies can be altered over time. (I really feel for the sister in your example!)
Bringing it back to pairs -- After Ashley Wagner stopped competing, someone asked her if she wouldn't consider trying pairs. She sort of shuddered and said that she could never trust a man that much. So, for the female partner, I think the ability to trust is a "sign" of pairs talent. And a corresponding ability for the male partner would be to create confidence by the way he lifts and throws the girl. He needs to communicate sureness, as well as respect and gentleness. The firm but gentle touch.
Another: some people only want, or are only able to depend upon themselves in any matter of achievement. Others like to be part of a duo or team, and thrive when a team can rise above what each individually would be able to accomplish. Those who believe that the whole can be more than the sum of its parts, and are able to translate that belief to their skating would have a great advantage in pairs or ice dance.