She succeeds without going that far, however. In fact, Miss Kesselman makes her vehement case without ever losing control of her craft. Though ''My Sister'' is full of grotesque events and shocking shifts of tone, its author negotiates most of her wild curves with ease, inexorably moving us into a crucible of psycho-sexual horror. Or so she does if one pays close attention to her words. At the Second Stage, where ''My Sister'' had its New York premiere last night, the production is at times out of sync with Miss Kesselman's text - despite the aid of an extraordinary performance from the young actress Elizabeth McGovern. The directors, Inverna Lockpez and Carole Rothman, eventually do catch up with the playwright, but not until the final third of the intermissionless, 85-minute evening.