THERE IS SOMETHING immensely moving about the plight of a grandmother threatened with jail for trying to keep her family together. So we cheered loudly the other day when the Supreme Court struck down the East Cleveland zoning ordinance that had made Mrs. Inez Moore, 63, a criminal for providing a home for one son, his child and a second son's child. Indeed, it seems outrageous that any locality would write its single-family zoning laws so narrowly as to make John Moore Jr., age 10, and illegal "unlicensed roomer" in his grandmother's home. Mrs. Moore could have sought a zoning variance, but went to court instead - and that is understandable, because it is doubly offensive to have local officialdom deciding, case by case, which types of extended families may live under the same roof.

On grounds of policy as well as compassion we are whole-heartedly with Mrs. Moore. It's hard to see how any legitimate community concern - such as overcrowding or noise or traffic congestion - can justify a zoning ordinance that, as East Cleveland's did, welcomes two parents with 12 children but outlaws an extended family of five such as the Moores. The case did not come to the High Court, however, on policy grounds. It came as a constitutional issue, and on that plane it is troublesome, because it involves some very hard questions about the limits of a community's power to promote certain values by rules that impinge on people's living arrangements and personal relationships.

So it's not surprising that the Supreme Court had considerable trouble with the case. The Court held for Mrs. Moore 5-4, without majority support for any single rationale. One complicating factor was that in 1974 the court had ruled that a community, to protect "family values, youth values" and local ambience, could bar occupancy of homes by more than two unrelated individuals. If Belle Terre, N.Y., could ban households of 5 college students, why couldn't East Cleveland bar the Moores? The vital difference, four members of the Court concluded, was that East Cleveland was "slicing deeply into the family itself."

The Constitution, Justice Powell wrote, "protects the sancity of the family precisely because the institution of the family is deeply rooted in this nation's history and tradition." Moreover, the tradition of extended families, "uncles, aunts, cousins and especially grandparents sharing a household . . . has roots equally venerable and equally deserving of constitutional recognition."

One can share this reading of history and tradition, and place equal value on families as a social foundation-stone, and still be uneasy about elevating protection of the family to the status of a constitutional command. As Justices White and Stewart indicated in separate dissents, this opens up a large assortment of very tough questions about which kinds of families and which aspects of family life might be exempt from regulation - and at what cost to other aspects of individual and community life. It is quite possible, for instance, that if the Court continues down this path it might someday find itself doing exactly what it has said that East Cleveland could not do - defining the "single family," and almost inevitably doing so in a way that discriminates against some group of close kinfolk somewhere in America.

The Moore decision, with six separate opinions, shows that the Court is still unsettled about what special weight to give to family values and relationships. That is encouraging; new constitutional doctrines of such portentousness should not be hastily proclaimed.In fact, we find it reassuring that the decisive vote for Mrs. Moore was cast by Justice Stevens in a way that circumvented the problem of the family's status altogether. The issue for Justice Stevens was whether East Cleveland's ordinance served the general welfare enough to justify its intrusion on her right to use her property at least points toward more solid ground by addressing the problem in terms of the rights of an individual - not the special status of a grandmother or grandson or any other relative.