He wasn't his guest, he was his brother. A little brother, and one who had lots of chatty things to say about Afghanistan at the White House last night. Like this:

"I didn't even know the place existed until the Russians invaded it," said Billy Carter. "Well, I did know about it, but I damn sure couldn't pronounce it."

Carter, 30 pounds slimmer and with a black tie at his throat, was pontificating on international relations, Bert Lance and country music at a dinner for the Italian premier. Funny thing, though. He said his brother never invited him.

"Tom T. invited me," said Billy, meaning Tom T. Hall, the country-western star who sang after dinner and figured he'd probably twang his guitar with the president, real late, upstairs by the fire.

Downstairs, watching the chocolate balls get passed around, little brother was still talking.

On former budget director Bert Lance's trial in Atlanta: "I think Bert will come free, myself."

On any connection between his Libyan pals and that country he can't pronounce to the east: "Afghanistan is 3,000 miles from Lybia."

And on country music: "I came mainly because Tom T. was playing tonight."

Meanwhile, the president had vanished. He was closeted away, conferring with Italian Premier Francesco Cossiga about nobody knew what. "They wanted," said Protocol Chief Abelardo Valdez, "to have some time alone with each other in private."

When he resurfaced, Carter smoothly dodged questions about whether he wants women to register for the draft. In Wednesday's State of the Union message, he had called for registration but avoided the gender issue.

"As you know," he said at the White House last night, "Congress decides that." Yes, but what did he think? "I will let the Congress know when the time comes," he replied. That time is Feb. 9, which gives him exactly 15 days to decide.

Rosalynn Carter told reporters that "if we register men, I think we're going to have to register women." But she added that "I'm not for drafting anyone."

Presidential aide Sarah Weddington said that although the president had decided to have Selective Service registration, "We have not decided how it would impact on women." She said Frank Moore's staff is "getting soundings" on the Hill.

Weddington said the president and Mrs. Carter were "very disappointed" over the 32 to 22 vote defeat by the Georgia State Senate of the Equal Rights Amendment Monday. She said that Carter spent the weekend making calls to Georgia for ERA and that in addition, several aides had gone there to work on its behalf.

"It means now we have to lay a strategy for 1981," she said.

While the president and Cossiga stood apart from the rest of the guests in the Blue Room, Secretary of State Cyrus Vance told reporters that he and Soviet Ambassador Anatoliy Dobrynin met for an hour and 45 minutes Tuesday night.

"We both told each other our views with respect to the situation," Vance said, referring to Afghanistan as well as the banishment of dissident leader Andrei Sakharov. "That's all I have to say."

Once a weekly luncheon visitor of national security adviser Zbigniew Brzezinski at the White House, Dobrynin has kept a low profile since returning from Moscow. Though he is dean of the diplomatic corps, he did not attend the president's State of the Union speech Tuesday with the rest of the diplomatic delegation, nor was he at the welcoming ceremony for Cossiga yesterday at the White House.

The subject of Italian-American relations shifted to focus on unity over the crisis in Asia. In predinner toasting, a Carter innovation at the White House, the president compared acts of terrorism: the taking of U.S. hostages in Iran and the murder of former Italian premier Aldo Moro.

"It's a significant measure of the character of a nation when it shows its response to an act of terrorism that hurts one person or a few people," Carter said.

Citing Italy's "absolute firm support" for the American position on the hostages, Carter said the Italian government also has expressed its condemnation "in very strong terms of unwarranted military aggression" by the Soviet Union in Afghanistan.

Obviously impressed with Carter's brief off-the-cuff speech, Cossiga decided to discard his own prepared text for extemporaneous remarks.

"I arrive at a time when the political situation of the world is in turmoil," he said through an interpreter. "I come as a friend among friends, an ally among allies."

An Italian official traveling with Cossiga's party said the premier and Carter discussed the U.S. Position on the Olympics yesterday.

"We still have not decided what we will do.We will consult with European countries in order to present a common front," the official said, adding that a decision was expected "in the next few days."

Cossiga arrived at the White House in a glistening snow-covered limousine, escorted by police and flashing red lights. He had been driven the distance of about two football fields from Blair House, the president's across-the-street-place for his guests.

Rosalynn Carter, who wore a green taffeta skirt and black velvet jacket, and the president waited inside where it was warm, until the last minute. When Cossiga arrived they popped out on the North Portico, got greetings over with quickly, then rushed inside as the U.S. Marine Band played "The Birds," by the Italian composer Respighi.

Most everybody at the dinner wore rustling evening gowns and black tie, except for The Storytellers, a country-western group that was part of the after-dinner entertainment. There were seven of them, looking straight Nashville in idential, western-cut blue polyester pants.

One, William Osment, was fascinated not by the White House memorabilia as most people are, but by the woman who played the harp for arriving guests.

"I figured out how she does it," he announced in the formal corridor for all to hear. "She's got different colored strings."

In his remarks during the toast Carter told the crowd, "One thing I've notice today is the rapid growth of Italian Americans." At Cossiga's arrival ceremony in the morning, Carter put that number at 7 million. Later in the day it had grown to 20 million, and by last night, Carter said, Rosalynn and fast-food millionaire Jeno Paulucci told him it was actually 30 million.

"I'm sure after the visit by the prime minister there will be at least 65 million," Carter joked.

In the East Room, where 110 after-dinner guests included five members of the D.C. City Council, Carter introduced country-music singer Hall and The Storytellers as exactly the kind of entertainment his guest of honor wanted.

"He said he wanted something truly American," said the president, standing against a background of 19th-century quilts, "and it didn't take us long to decide.

After the music, after the midnight champagne, after the Marine Corps played "Tonight" and "America" from "West Side Story," Carter said goodbye to his guests and returned upstairs.

And with that, the band struck up something else truly American: "Iowa Corn Song."