If the worry truly is for this other, mortal brother.. "Hm. If the advice of a stranger is worth anything, bide your time and see where it develops. While accepting new family out of the blue can be a struggle.. many people find it much easier to accept a new friend first."

Granted his experienec with it isn't overwhelming or anything, but once was enough. What would have happened, where would he have been if Jonathan hadn't been willing to extend even that branch of friendship? "Especially in a situation like this, where all else is also new and strange. ... Though if this one common person is in both your histories, it might also be wise to prepare in case this someone arrives as well. The train won't let you kill them. Permanently."

That last word is important. Instead there's opportunity to kill them over and over and ove--

Wait no that's not the point even if it's the most appealing part.