THE ELDEST OF five sisters, I looked forward to reading Elizabeth Fishel's book for the same reasons that I would pick up a book on astrology devoted to my own star sign:

Partly for the self-indulgent pleasure of reading about myself, of seeing my identity confirmed in print, and partly in the hope of discovering some new perspective on that identity.

It happens that Fishel's prime motivation for writing her book was not very different from mine for wanting to read it, for it was from the particular vision of her own sister, Anne, as "my dearest friend and bitterest rival, my mirror and opposite, my confidante and betrayer, my student annd teacher, my reference point and counterpoint," that she set out to prove, by the time-honored process of induction, the general importance of sisterhood.

Her first step was to place advertisements in both Ms. magazine and the Mothers of Twins Club's newsletter. Her second, to send questionnaires to all those readers who expressed a willingness to answer her questions on their relationship with their sister(s). Of the questionnaires sent out, over 150 were returned completed; which, Fishel concludes, "testified to the depth and breadth of feeling women had about their sister." It would, nonetheless, have been interesting to have been told what percentage of the two periodicals' readership the figure 150 represents: in other words, how many readers did not consider that their feeling about their sister(s) was worth the bother of communicating.

Of course, substantial (and maybe incalculable) numbers of Ms. readers and Mothers of Twins Clubs members must not have sisters about whom to write, but that brings me to the second, more serious flaw in the usefulness of Fishel's reseach methods. In attempting to establish the importance of the sister relationship, she fails to establish a control group; to enquire, for example, into the feelings of girls with all male siblings for their brothers, or the nature of the relationship between only, female children and their extra-family girl friends.

The omission might not be worth mentioning were Fishel's thesis merely that sisterhood is, in some indefinable way, important. But neither, if that alone were her thesis, would the book have been worth writing, no proposition so truistic needing some 200 pages of exposition. Instead it must be assumed that, in the answers to her questionnaire and in the 50, subsequent, more exhaustive interviews, Fishel was attempting to discover some common factor; to define sisterhood's specific nature.

But what emerges most strongly from both the questionnaire replies and the interview transcripts (of which, if should be said, many are perceptive, articulate and, as individual stories, fascinating) is the diversity of feelings that exists between sisters and sisters: some considering themselves as one another's best friend, some loving one another incestuously, some claiming to have a parent/child relationship, some hardly seeing one another from year to year, some united by hate.

Nor is the intensity with which these various emotions are expressed of similar concentration (particularly, if is interesting to note, amongst the celebrity interviewees whose attitude, having been sought, not volunteered, might possibly be less partial).

Exposed, sisterhood is seen to be a condition open to endless variation:

differing age gaps, the presence or absence of brothers, parental attitudes, birth order and myriad other factors capable of altering its nature. When Gloria Steinem was approached by Fishel for an interview, although agreeing to give one, she said, "We [her sister and herself] really are not typical. Our stories would not be that helpful to others, because we're so far apart in age, more like mother and daughter than most sisters." Her comment on atypicality applies to all the respondents.

There are, it is true, themes that recur. Jealousy, role playing, telepathic communications, the sharing of a private language and/or private jokes are apparently common attributes of sisterhood. But they are not the prerogatives of sisterhood, being equally applicable either to family members in general, or to friends, or to any small group united by special circumstances.

In the end, then, sisterhood remains as undefined a state, as personal a concept, as intuitive an idea as it was at the beginning. Too vague in its intentions and both too vague and too speciic in its procedure, Fishel's book leads us nowhere. A concluding chapter on "Sisters Outside the Family" is based partly on interviews with the poet Ntozake Shange and her sister/friend Thulani and partly on interviews with members of a feminist commune. It completes, in its appearance of having been appended for no particular reason, the reader's disappointment.

Though containing some interesting, individuals case histories, though permeated with the author's sincerity and enthusiasm for her subject, Sisters amounts in the end to little more than the horoscopic prediction: "Tomorrow you will probably hear some news." Unarguable, but useless. CAPTION: Illustration, "The Twins," by Aubrey Beardsley