I'm actually digging through the forum for open/pending questions/problems. And I've seen, there aren't any answers to Your Post.
Allow me to explain the Family Group Sheet:
A family in the genealogy context allways contains 1 husband, 1 wife and the children of this couple.
If, for instance, this husband married a second wife, this is from the system view another family. In the FTB person card You can see the other wifes as a linkto the other wife. So, if You want to generate Family Group Sheets, You must build the first one (with first wife), and the build the second one (with the second wife).
As You can see, the title of both sheets are different.