are they more upset by the Manuel treats Sofia or that Sofia might stay on the island indefinitely to be with her boyfriend??

I think they are more upset about Manuel than anything else. The three elder sisters have concocted a scheme to separate the teenage Sofía from her cousin, Manuel Gustavo, whom she has been dating while living in the Dominican Republic. Earlier in the chapter, the sisters compare Manuel's domineering behavior to tyranny, implying that Trujillo's dictatorship gains some of its staying power from the patriarchal culture the expects masculine dominance and demands unquestioning submission from women. Trujillo fulfills the traditional model of a macho man, but Manuel's pettiness and ignorance indicate how superficial and uninspiring that masculine archetype is when viewed critically.