Solidarity leader Lech Walesa, his wife Danuta and their seven children have been reunited for Easter in the villa outside Warsaw where he has been held by martial-law authorities for nearly four months, sources said yesterday.

It was believed to be the first time Walesa has seen his entire family since he was interned along with thousands of Solidarity union activists when martial law was imposed Dec. 13. Walesa's seventh child, a girl, recently was baptized in Gdansk, but he was not allowed to attend.

Family sources said Walesa's wife and children would stay with the union leader through Tuesday and then return to their home in Gdansk.

In another development, sources said that a third Polish ambassador has defected to the West and is believed to have sought political asylum in the United States.

Wlodzimierz Wisniewski, 56, a former ambassador to Finland, was replaced in Helsinki about two months ago, but subsequently disappeared.

A Polish government source confirmed that Wisniewski had failed to return, adding that officials here had become suspicious, investigated what was supposed to be a shipment of Wisniewski's belongings awaiting shipment from Helsinki to Warsaw, and found the crates to be empty.