THEY are siblings, only a year apart in age but light years apart in status and accomplishment. He is rich; she is poor. He earned an Ivy League law degree; she never went beyond high school. He often travels in chauffeur-driven limousines; she looks out her front door at an old car on blocks. He moves in a world of the famous and powerful. She shuttles between her tumbledown two-room house and her job as a hospital cook, where she sometimes begins work at 3 a.m.Since President Bush nominated Clarence Thomas to the Supreme Court earlier this month, reporters have written with a mixture of fascination and bewilderment about the nominee's older sister, Emma Mae Martin. They have quoted - and quoted again - Mr. Thomas's harsh criticism of his sister for accepting welfare a decade ago. And they have skirted around an unstated question: How can he be so ambitious and successful and she so content to remain in a hardscrabble Georgia backwater? Their lives began taking divergent paths in 1955, when the young Clarence and his brother went to live with education-oriented grandparents and the young Emma Mae and her mother moved in with an aunt. Although all three children attended the same parochial school, who can say what vastly different messages, spoken or implied, teachers imparted to students of that generation, perhaps encouraging careers for boys and domesticity for girls? And who knows what disparate roles and ambitions were encouraged in the Thomas children's respective homes? Even today, in supposedly egalitarian middle-class households, daughters and sons often receive very different treatment. According to a recent study by two Arizona sociologists, teenage sons in dual-career families spend less than three hours a week on household chores, while daughters spend 10 hours. This inequality only reinforces deeply rooted notions that men's education and careers are more important than women's, and that men are above domestic tasks. Like her mother before her, Emma Mae Martin was a teenager when her first child was born. Her husband deserted the family in 1973, moving out of state to avoid paying child support, she claims. In addition to providing for her own young children, Martin also assumed the care of her elderly, incapacitated aunt. Had it not been for those care-giving responsibilities, which forced her to quit her job, and the absence of child support, Martin says she would not have accepted welfare. She went back to work - and off welfare - after her aunt's death. Publicly, Martin seems remarkably sanguine about the road not taken - the higher education that could have taken her out of Pin Point, Ga., into a larger, less-impoverished world. "I had the opportunity to go to college if I wanted to, but I made the choice," Martin has said. "I took care of the older people." Her experience illustrates the challenges many women face in remaining self-sufficient while caring for children and older relatives. Bootstrapping - a word Clarence Thomas proudly uses to explain his own success - becomes more difficult when a bootstrapper has many dependents and care-giving duties. A new survey by the Older Women's League shows that three-quarters of all care givers to the elderly are women. Four out of five are parents, more than half of whom have children under 16 still living at home. Nine in 10 report that their care giving diminished their ability to work in the same way they did before assuming elder-care responsibilities. As for child support, every year more than $6 billion in court-ordered payments go uncollected, sending more children into poverty and their mothers onto welfare rolls. New laws and more aggressive collection methods are improving the situation, but more than two million absent parents still evade their responsibility annually. Birth control now gives women more reproductive choice, and employment opportunities offer them greater economic independence. What has not really changed is the social attitude that tends to give hierarchal preference to the education and career of a brother at the expense of a sister. As long as men's work and women's responsibility remain so arbitrarily defined, the brother will give the Supreme Court decisions and the sister will give the hands-on care - and be judged as the family failure to boot.