This is a topic would occasionally make my head hurt back when I was a believer.
If you were to ask a young Mormon child “who is your eternal family?” they would likely say something like “me, my mommy, my daddy, my brothers, and my sisters.” If you were to ask someone with young children the same question, they would likely think of their spouse and their children. The problem is that TBMS tend to see an “eternal” concept as a frozen-in-time snapshot. Children grow, marry, and have children of their own. Your spouse might very well be “sealed” to his or her parents and already be part of an eternal family with parents, siblings, aunts, uncles, and in-laws. Your own parents are not only sealed to each other, and to you, and to your siblings, but also might be sealed to their parents, and grandparents, and so forth. This leaves only two possibilities:
1) Eternal families in the CK are massive--with mixtures of people across many generations with countless connections from both blood and marriage relationships, or 2) Eternal families only consist of a husband and one or more (likely more) wives.
(3) Mormonism doesn't/ Mormons don't know the meaning of eternity: without beginning or end. You can be married or sealed for eternity because that would include spirit coming into the body at birth, and so on... thereby making the ordinances and ceremonies absolutely and unnecessarily useless and baseless.
The Mormon concept of an "eternal family" is a family frozen in time--souls aggregating like limestone, up there somewhere.
IMO a family is a dynamic thing. People die, people are born; the family of 40 years from now will be very different from the family of today, but it will still be "family." Maybe one or two of today's young ones--my grandson, maybe--will sit there at Thanksgiving dinner and talk about Grandpa getbusylivin, and how Gramps loved his Coca-Cola and his cats, and had some awesome tattoos. And then the conversation will shift to somebody's brother or son or my greatgranddaughter's plans after college, and I'll slip farther and farther from the collective memory into eternal oblivion, which is as it should be, in my opinion, because the family should be ALIVE, not DEAD, like the quote-unquote "eternal family."
One thing that has dawned on me, is how far back do you go in your family history before the relationships become interesting, but essentially meaningless? Where the only thing that you have in common is a rather weak genetic link? I think the answer is different for every person and every family, but I suspect that once you get past the first couple of "greats," the ties begin to loosen. I mean, how many people (in theory) could go to George Washington, calling him "(x)Great Grandpa," before he would just shrug his shoulders and respond, "That's nice"? Think about the number of grandchildren and great-grandchildren in large Mormon families who are even now just one of a crowd.
Family history to me is interesting because it is a personalized version of history. But how much would I have in common with these ancestors, exactly?
I have come to the conclusion that what we pass down is (hopefully) a chain of love -- that you give love to your family members, who in turn give love to their family members, etc. We may not have much in common with our descendants five or six generations from now, but we can pass down that sense of love, responsibility, and devotion.