An exhibit of promptbooks, costume designs and other memorabilia belonging to the Kemble theatrical family -- J.P., Charles, Fanny and Sarah Kemble Siddons -- of the 18th and 19th centuries will go on display at the Folger Library Thursday. It's open every day from 10 a.m. to 4 p.m. except this Sunday, through October . . .The Folger Theatre is having a benefit Saturday at Bloomingdale's (White Flint), linking the store's display of Style Scandinavia with the theater's current production of "Hamlet." It is said the Prince of Denmark will meet Princess Christina of Sweden at some point during the evening. Call 547-3230. . .

New Playwrights' Theatre is having another benefit tonight, this time a beer tasting, hosted by Mark Weiner of Berose Liquors. It costs $10. Call 232-1122. New Playwrights' also received its largest corporate grant to date, $8,000, from Exxon, thanks to Donald E. Smiley and Monika Bernstein in the Washington office. That brings the theater's total to $116,000 of the $250,000 fund-raising campaign goal. . .

Jack Lee, the musical director of "My One and Only," is holding a symposium in vocal techniques for the musical theater tomorrow at the Kennedy Center. Call Todd Lincoln at New Playwrights' (232-4527) if you are interested in being either a participant or an observer. . .

There is at least one mother-daughter combo performing in the area: June and Elizabeth Hansen. June has played at the Arena, Olney, the Round House, the Folger and was in "The Physicists" at the Kennedy Center. Her daughter Elizabeth, 26, is a graduate of the Professional Actor Training Program at the University of North Carolina and is currently performing her first semiprofessional role at the Studio, as the parlormaid in "La Ronde" . . . .