I don't know. I am not expecting a harem here. While I think this is one of those few stories in which a kind of harem could work*, I am actually waiting for the moment in which we will have to pick.
*The reason why I comment I could see a harem working here is simply because the setting allows it. In some sense, the key element is Cordia and the hierarchical structure of the family, mixed with the fact that Deluca is a family of wierdos.
Let me explain myself better. We have first that everybody obeys Cordia and her word is law. We have then that it was suggested (maybe not, but it was what I understood) that Cordia has remained alone since the death of Lucan. Provided her kiss was not motherly, she could developed feelings for MC.
Now, I can see a situation in which she would allow her daughters to be romantically involved with MC (it is again hinted she is already allowing it, when Wilfred comment to MC they are letting him have his way with the girls, at least for now).
Only way in which I can see Cordia also allowing Isabel's to be involved with MC is if Antonio died. The reason is that which mother would want her son to be cheated and I don't think the whole "Antonio goes gay" will ever happened.
So, all in all a situation in which Cordia, Luna, Gracie and Isabel date MC is potentially possible. First, however, let me be explicit here. I don't see a harem in which MC goes a pick which girl he wants to fuck and all other are happy and might even join. I see a situation in which each girl is aware he is dating the others, but nothing else. Second, it is Cordia who would be the harem queen, not MC, but simply it would be MC the one benefitting from it. This last part again follows from the hierarchical structure.
In general, and even with these variations, I would be very against the idea, as everybody agreeing MC can fuck everybody is a terrible way to write an interesting novel, as it kills a lot of potential conflict. Luckily, Deluca's plot is not a pure love story. The driving story is not about getting the girls. That is one of the things going on, but more a consequence than the cause of the plot. So even if eventually all relationship conflict was potentially killed, I can see the novel having so much things going on that it would not be truly detrimental.
HAVING SAID ALL THAT, I am still not betting money on this being the case. We will most likely have to pick, eventually. Or it might end up being one of those novels where the relationships issues are never resolved, in which the novel finishes with MC unable to pick.. One way or the other, we'll see, but I suggest you lower your harem expectations, though.