That doesn’t mean that in time the sibling can’t romance them both now, eh? 
In the sovereignty ending the identical sibling does make it easier for the mc to sneak out with Jack on occasion, particularly if they can effectively share ducal duties with their identical sibling on occasion, much like how in some of the possible endings of the Hero Unmasked that mc’s sibling is perfectly capable of hosting his brother’s talk-show without (m)any people being the wiser.
In the abdication ending their lives would be more separate as I don’t think little bro will move permanently to the vineyard with Jack and my mc, instead likely becoming the new city-boy.
If you do get the opportunity I can highly recommend it, for me only the very first/pilot episode was a bit uncomfortable to watch, it gets smoother from there, wereas, again, TNG took two whole seasons to really hit its groove.
I’ve a more elaborate comment on the TV shows thread on it.