victor and elizabeth were not the first grooming case nor the first pseudo-incest relationship in frankenstein: that would be alphonse and caroline.
alphonse was a friend of caroline’s father, beaufort. this is how they met, and so there was a significant difference in their ages. after beaufort dies, alphonse and caroline marry. take a look at how beaufort’s passing is described:
Her father grew worse; her time was more entirely occupied in attending him; her means of subsistence decreased; and in the tenth month her father died in her arms, leaving her an orphan and a beggar. This last blow overcame her; and she knelt by Beaufort’s coffin, weeping bitterly, when my father entered the chamber. He came like a protecting spirit to the poor girl, who committed herself to his care, and after the interment of his friend he conducted her to Geneva, and placed her under the protection of a relation. Two years after this event Caroline became his wife.
while "orphan" does not strictly mean the person is a minor, orphan still is most commonly used to describe a minor whose parents are both dead. if we interpret orphan in that sense, then caroline would have been a child when alphonse first took her in. the fact that he waits two years after this event to marry her also hints towards this, almost as if he was waiting for her to become legal and the age of consent. this is further supported by the diminutive language of “poor girl” used to describe her, who is in juxtaposition to the paternal “protecting spirit” of alphonse whom she commits herself into the care of.
even if caroline was not a minor, there was a large enough gap in their ages - and the fact that alphonse “saved” caroline from poverty, creating an economical reliance on him - that there was an unhealthy power balance in their relationship. because of this dynamic, it really does read like grooming: alphonse houses caroline till she is (supposedly) old enough to marry, and by that time she would have been pushed into consenting to the marriage because she relied on him for money and housing, and could have some sort of emotional obligation to him as well for supporting her in a time of need and grief, and he is a significant link to her deceased father. this difference in their ages is highlighted again when victor notes that alphonse was in the decline of his life by the time he and caroline were having children together, and by the time victor is 19 alphonse is old enough that he is physically incapable of traveling to ingolstadt.
in this way their relationship is pseudo-incestous, because alphonse (her father’s age) swoops in to support caroline (a child) after her father dies. this makes himself the father figure replacement, and caroline his daughter. once she is of age she transitions from the role of daughter to wife, and during her marriage caroline will go on to repeat this cycle of abuse, and recreate this same dynamic - except this time, it is in a situation that she can control: through victor and elizabeth.
from the beginning caroline deliberately sets up parallels between herself and elizabeth. she wants a daughter, and adopts elizabeth specifically because elizabeth reminds her of herself, but grander: like she was, elizabeth is also a beggar and an orphan and homeless, but her story is more tragic, she is more beautiful, her debt to her caretakers more extreme, and her romantic relationship will go on to be more explicitly incestous. caroline calls elizabeth her favorite and grooms her into becoming a second version of herself, so that she can recreate the traumatic event of her marriage with her two children.
so, as caroline dictates the marriage between victor and elizabeth, victor becomes to elizabeth what alphonse was to caroline: a man, who is also a familial figure, that she must marry in order to have a stable social and economic life. the frankensteins have provided elizabeth with everything she has, and the threat is there that they can also take it away if she does not comply (through marrying victor), which is the same kind of looming, unspoken threat that hung over caroline and alphonse’s marriage.