In Mary Shelley's Frankenstein, Victor Frankenstein relates his history to Captain Walton in Chapter One. There he tells the sea captain about his family of whom he was the only child. On a holiday to Lake of Como in Italy, he kind mother who visited the poor noticed a poor peasant woman "distributing a scanty meal to five hungry babes," one of whom was strikingly different from the dark-eyed "little vagrants." She was blond, blue-eyed, and of a visage of "sensibility and sweetness." The peasant woman told Mrs. Frankenstein that the girl is "the daughter of a Milanese nobleman" who may have been imprisoned in Austria and the mother had died giving birth. Thus, Elizabeth was left orphaned. The tender-hearted Mrs. Frankenstein prevailed upon the guardians to "yield their charge to her." So, Elizabeth and Victor grew up together in a loving relationship. Fondly, the family refer to her as "cousin," but there is absolutely no blood relationship between Victor and Elizabeth! His only blood relative is his young brother William.
It is because they were children together in the same household that Elizabeth wonders if Victor perceives her only as a sister:
We were affectionate playfellows during childhood, and, I believe, dear and valued friends to one another as we grew older. But as brother and sister often entertain lively affection towards each other without desiring a more intimate union, may not such also be our case?
After having seen him last autumn when he returned home, but avoided contact with people, Elizabeth worries that Victor might "regret [their] connection" while feeling bound to the wishes of his parents. She does not want Victor to marry her unless it is the "dictate of [his] free choice." She is worried that Victor will feel bound by honour to marry her and stifle "all hope of that love and happiness which would alone restore you to yourself."
So, Elizabeth worries that Victor
- May not wish to become more intimate than the relationship of brother to sister that they had as children
- May feel honor-bound to fulfill the wishes of his parents and not choose her of his own free will.
- If he does marry Elizabeth because of honor, he will not return to good health because only true love and happiness can do this.
I assume that you are talking about what Elizabeth says to Victor in the letter that she wrote to him -- the one that is shown in Chapter 22. If so, I think that the other reason that she thinks he might not want to marry her is because they were so close as they were growing up.
Victor and Elizabeth grew up together almost as if they were brother and sister. They were, in fact, cousins, but had a closer relationship than that as they grew up. Because of that, Elizabeth thinks that Victor may not want a "more intimate union," as she puts it in the letter.