THE LAW HOLDS, and proponents of reform agree, that in deciding which immigrants to admit to the United States, preference should be given to those with close relatives already here and those with special training or skills. But which close relatives? In the Simpson-Mazzoli immigration bill, which has passed the Senate and will soon be considered in the House, public attention has focused mostly on provisions dealing with illegal aliens, refugees and sanctions on employers who hire undocumented workers. The emotion-laden question of who is considered family for purposes of the law must be considered, too.

Almost everyone would agree that spouses and minor children of both citizens and permanent residents should be given priority, and they are. So are citizens' adult children, though their priority is lower than that of minor children. And so are the parents of adult citizens, who are often dependent financially and emotionally on their grown children. But what about brothers and sisters? Under present law, all siblings of citizens have a preference. The bill the Senate has passed would eliminate it. This was not a cold, anti-family decision, but a difficult choice made to favor some family members over others when only a limited number of places is available.

In the category of brothers and sisters (the "fifth preference"), a backlog of more than 700,000 people is waiting for visas. The group is large because it includes, of course, all the spouses and minor children of the siblings who have an automatic right to come along. In one case, a single citizen petitioned for visas for all her brothers and sisters, their spouses and their children: 69 people.

But there is also a backlog of applicants for "second preference" visas -- the spouses and minor children of permanent residents. Since the new law would limit the total number of immigrants admitted each year, the Senate decided to make more places available to second preference applicants and to eliminate the preference for brothers and sisters except for those already on the list.

Sen. Kennedy proposed a compromise to give preference only to unmarried brothers and sisters of citizens, but it was defeated. This compromise, however, and an amendment increasing the overall quota by 25,000 visas a year, are in the bill that the House Judiciary Committee is expected to take up soon.

It would be nice if grandparents and cousins and aunts and nephews could all come together to America as families did a hundred years ago. But with a fixed number of places available, priorities have to be assigned. There is merit, not to speak of emotional weight, to the arguments of both those who want to preserve fifth preference and those who would eliminate it in favor of other family members. In a situation like this, compromise -- even a time-limited one -- makes the most sense so that an otherwise generally practical and thoughtful reform can go through.