Okay, so I'm thinking that the reason for the mother's favouritism is that the spirits told her
that her first born son would be an incredibly talented leader and swordsman, but her firstborn who was supposed to be said son didn't end up being so, thus changing fate and making the prediction wrong.
Another thing I noticed is that it sounds like the mother actually ignored the second son in the original plot, so that kid is kind of pitiful both in this story, and in that story too.
So I think the mother wasn't bad or negligent, but merely trying her best for the supposed fate of her children, but just in the very sloppy way.
As for this prince, and why he said 'do you feel anything?'... my first thought that he might be the original brother, who is now also isekai'd to this world, but since that'd be creepy as hell since he appears to be the love interest, that better not be the case.
He could also be the original Older Son of the MC's family, but then that leaves the question of what happend to the original prince's soul.
A normal isekai person would likely just be confused by a daughter instead of a son existing, so that only leaves a person who
]met her in the past and was so charmed that he wants to increase his own power to win her over to him[/ISPOILER - which I think is the most cliche option, but the only one that makes sense currently. 