BY WAY OF SHOWING my credentials for dealing with the issue of Billy Carter, I'd like to cite my former relative, the buxom and outrageously blonde Shirley. She passed through our family like Halley's Comet, suddently marrying in, rather quickly divorcing out, and in the meantime shocking us with plunging necklines, a frank manner of speech and an occupation that involved ladies who came out of cakes naked. While she was with us, we defended her to outsiders. She was, after all, family.

Shirley -- louid, brassy shirley -- was just one example. I come from a family whose pioneer immigrant to these shores did time for mistaking manhole covers for scrape iron and selling them during World War I at what I take was a good price. He does not hold a chandle, however, to another relative who died, or so it was said, of an overdose of rancid farmer's cheese -- and just how he would do pitte against my wife's relative, the one who drove his car across a neighbor's lawn, is beyond me.

The point is that we all have come relative who it totally out of control, some person who is the subject of giggling dinner conversations whenever the family gathers. Presidential families are no exception. Richard Nixon was burdened with brother Donald and Eward and they, for that matter with him, and Lynodon Johnson had as a brother the infamous Sam Houston Johnson. He claimed to have been held prisoner in the White House and went on to write a book that made a strong case for his incarceration. If presidents are not high enough on th social scale, just think of the laughs Margaret must have given her sister, QUEEN Elizabeth. What we all do, no matter who, is draw up the wagons, deal with the situation as best we can and keep it in the family. Some things, my parents used to tell me, you do not tell your friends.

But Billy Carter is a special case. He is special because he is the first presidential relative to make a living as nothing more than a presidential relative. Margaret Truman's singing career might have been helped because her father was the president, and Susan Ford does not push suburus because she's well-known in the auto biz. But Billy Carter is the first presidential relative who does nothing more than trade on his relative's name. If you asked him what he does for a living he would have to answer that he makes a buck being the brother of the president.

For a while all this was kind of cute. The prsident himslef seemed to get a kick out of his brother and he told reporters in the early days to go down and see Billy if they wanted an insight into Jimmy. Sister Ruth Carter Stapleton weighed in with a book, "Brother Billy," in which she said his redneck act was just an act. Billy, his sister told us, is not only a canny businessman, but a book reader as well.

All of this tends to enhance our interest in Billy. He is the brother, after all, of one of the most inscrutable men ever to hold th office of President and what fascinates about Billy is what we think he tells us about his brother. Somehow you get the notion that Billy is supposed to be the essential Carter, what Jinny would be if the had not goen into politics and found religion along the way. To that extent, Billy humanizes Jinny, takes him down a peg, lets you know that our president knows a curse word ot two.

So now we come to Billy to Billy Carter and his recent remarks about his brother's aides and about his own new-found friends, the wounderful people who helped bring us the Munich Massacre -- the Libyans. The White House would like us to see daylight between Jimmy and Billy Carter on this, share his views. What we are being told, more or less, is that this is family and you know how that is.

This is a temptation to quibble. This comes, after all, after the president has managed to blur this distinction between his office and his family. He sends family members on official trips, using them as if state visits were family obligations and he has, i nthe past, merely chuckled when Billy proved time and time again there weas almost nothing he would not do for a buck -- yuk, yuk, good 'ol Billy.

But having said that, we still all know that Billy is not Jimmy, that they do not share the same views, and that to hold Jimmy Carter responsible for his brother's antics is a wild extension of the imperial presidency. There is a limit to what a man can control -- the sad case of Martha Mitchell being an exampl of that. Billy Carter is not an appointee, not even a friend; he is the man's brother and what he probably needs, more than a public rebuke, is help. That's what big broters are for.