We found a detailed article on the Web site of a Louisiana TV station by Chris Elliott, a regular contributor to the Inquirer's Sunday travel section, about the ways airlines may wind up charging families extra so they all have seats together. US Airways and its policies are just the lead of the story since most airlines now charge extra for certain seats. Read all about it froim Elliott's perspective here. This is just one of many "unintended consequences" of the airlines' love of ancillary revenue from fees for services, prodcts or privileges once included in your air fare.
One aspect of this topic that gets short shrift in the article is to call Southwest "iffy" in terms of being able to seat all members of a family together because the airline has no assigned seats. Seems to us that most families have a very good chance of finding seats together on Southwest if at least one person checks in online early, or pays an extra $10 to get priority boarding (a true "convenience fee" that has proved well worth it in my experience). The one checking in early is in the "A" boarding group, the first allowed on a flight, and can then hold seats for other family members. If the family members are on separate tickets, each could check in early. Even a family that needs to stick together and is put at the head of the line of the "B" boarding group can probably find seats together, since each group holds only 60 people, and the airplanes hold almost three times that many.
Let us know what your experience has been with finding seats together for all family members.