(Spoilers for dmc 4-5) what does dante mean by the yamato "reacting" to nero?

During 5, dante says to nero that he won't let him go kill his father (or something among those lines). He says he had a hunch before that nero was vergils son but after seeing how the yamato reacted to nero, that he was certain. What does he mean by that? I know it's said that rebellion and yamato were given to dante and vergil but did the weapons chose them and the yamato reacted to the sparda bloodline? Or did the yamato react strongly to nero because it was previously bonded to him in the form of his devil arm?