The girl is spending the summer with her extended family. She doesn't put it this way. But as we talk on the beach, the 10-year-old lists the people who are sharing the same house this month with the careful attention of a genealogist.

First of all there is her father--visitation rights awarded him the month of August. Second of all there is her father's second wife and two children by her first marriage. All that seems perfectly clear. A stepmother and two stepbrothers.

Then there are the others, she slowly explains. There is her stepmother's sister, for example. The girl isn't entirely sure whether this makes the woman a step-aunt, or whether her baby is a step- cousin. Beyond that, the real puzzle is whether her step-aunt's husband's children by his first marriage have any sort of official relationship to her at all. It does, we both agree, seem a bit fuzzy.

Nevertheless, she concludes with a certainty that can only be mustered by the sort of 10-year-old who keeps track of her own Frequent Flier coupons: "We are in the same family." With that she closes the subject and focuses instead on her peanut butter and jelly.

I am left to my thoughts. My companion, in her own un-self-conscious way, is a fine researcher. She grasps the wide new family configurations that are neglected by census data takers and social scientists.

After all, those of us who grew up in traditional settings remember families that extended into elaborate circles of aunts, uncles and cousins. There were sides to this family, names and titles to be memorized. But they fit together in a biological pattern.

Now, as my young friend can attest, we have fewer children and more divorces. We know that as many as 50 percent of recent marriages may end. About 75 percent of divorced women and 83 percent of divorced men then remarry. Of those remarriages, 59 percent include a child from a former marriage.

So, our families often extend along lines that are determined by decrees, rather than genes. If the nucleus is broken, there are still links forged in different directions.

Last winter the son of a friend was asked to produce a family tree for his sixth-grade class. But he was dissatisified with his oak. There was no room on it for his step-grandfather, though the man had married his widowed grandmother years ago.

More to the point, the boy had to create an offshoot for his new baby half- brother that seemed too distant. He couldn't find a proper place for the uncle --the ex-uncle to be precise--whom he visited last summer with his cousin.

A family tree just doesn't work, he complained. He would have preferred to draw family bushes.

The reality is that divorce has created kinship ties that rival the most complex tribe. These are not always easy relationships. The children and even the adults whose family lives have been disrupted by divorce and remarriage learn that people they love do not necessarily love each other. This extended family does not gather for reunions and Thanksgivings.

But when it works, it can provide a support system of sorts. I have seen the nieces, nephews--even the dogs--of one marriage welcomed as guests into another. There are all sorts of relationships that survive the marital ones, though there are no names for these kinfolk, no nomenclature for this extending family.

Not long ago, when living together first became a common pattern, people couldn't figure out what to call each other. It was impossible to introduce the man you lived with as a "spouse equivalent." It was harder to refer to the woman your son lived with as his lover, mistress, housemate.

It's equally difficult to describe the peculiar membership of this new lineage. Does your first husband's mother become a mother-out-law? Is the woman no longer married to your uncle an ex- aunt? We have nieces and nephews left dangling like participles from other lives and step-families entirely off the family tree.

Our reality is more flexible and our relationships more supportive than our language. But for the moment, my 10- year-old researcher is right. However accidentally, however uneasily, "We are in the same family."