This isn't the real Mr. Sinister, though, it's an unstable mutant clone that's also a gossip columnist.
I also don’t think Magneto KNOWS that Mr Sinister used to work for Nazis. He was pretty covert if i recall correctly
Things will probably get bad when Hickman fires the Chekhov's gun that is Magneto's death. When he comes back, Charles might not want him to go the radical route he proposes and it will go downhill from there.
Wolverine knows, because he got the goods on Essex's involvement with weapon plus, way back in New X-Men. They recruited Sinister because of his 'good work' over in Europe. If anything, Magneto should be apoplectic that the dude jumped straight from human experimentation to carving up mutants. Moira was completely right to want to avoid him. He's not even a mutant.
"A happy ending? So unlikely. We're not having a moment here.
Wrong city, wrong people, all huddling in fear.
No one escapes the slaughterhouse, and that's just where you're at.
(You could've asked Rebecca but then Adam stomped her flat.)
You think you're special cuz you're scrappy? You're deluded, time to go.
Lucy's living on the moon but you're another dead psycho."