how is the family described?

THe actual stage directions pretty well sums answers your question. 

At rise of curtain, the four BIRLINGS and GERALD are seated at the table, with ARTHUR BIRLING at one end, his wife at the other, ERIC downstage, and SHEILA and GERALD seated upstage. EDNA, the parlourmaid, is just clearing the table, which has no cloth, of dessert plates and champagne glasses, etc., and replacing them with decanter of port, cigar box and cigarettes. Port glasses are already on the table. All five are in the evening dress of the period, the men in tails and white ties, not dinner jackets. ARTHUR BIRLING is a heavy looking, rather portentous looking man in his middle fifties but rather provincial in his speech. His wife is about fifty, a rather cold woman and her husband's social superior. SHEILA is a pretty girl in her early twenties, very pleased with life and rather excited. GERALD CROFT is an attractive chap about thirty, rather too manly to be a dandy but very much the easy well-bred man about town. ERIC is in his early twenties, not quite at ease, half shy, half assertive. At the moment they have all had a good dinner, are celebrating a special occasion, and are pleased with themselves.