Thinking about the suggestion that James acts like a jealous husband towards Clarissa, it is true that in earlier periods the oldest brother was seen to have a special position in the family not only because he was the heir, but as a secondary authority in the family to whom the women could and were supposed to apply for various kinds of help (in going to law, about jointure especially, in women's dealings with their husbands after they left their own family & were with the husband's family). I have seen this kind of "special relationship" between sisters and their oldest brother in more than one Renaissance family: in the case I know well when Vittoria Colonna's husband died, she turned again and again to her oldest brother, Ascanio; sometimes it seems to me he wishes he didn't have to respond but it's his duty; he also tries to enforce the family's will in trying to get her to remarry; how long such a feeling lingered is hard to say.
Also as I remember reading (probably in Robin Fox's book on kinship systems) about so-called matriarchies where the property is inherited through the female, what happens is that the oldest uncle (i.e., the woman's brother) assumes a strong role in the clan, and a strong say in what the woman does (who she marries &c). Now brothers instead of fathers trade women. But perhaps someone has a more precise idea about this area of anthropological studies than I do.
I think later in _Clarissa (somewhere someplace in this vast book) there is a passage where James talks about his "prerogative;" and in Letter III we have already seen Mr Harlowe defer his decision on Lovelace until James returns as follows:
he had a letter from his son ... which he had ot shown to anybody but my mother ... in this letter [James] expressed great dislike to an alliance with Mr Lovelace ... he would suspend the declaration of his own mind till his son arrived ...
The reason cited is a desire for harmony among them all; but James is decided given an option to refuse as if he had a natural right to take the father's role.