I don't know what the fuck is going on but I'm here for it. Arthur just CASUALLY has siblings now after his parents deserted him lol. I'm guessing Joker was referring to Arthur's parents.
I don't care how much of a joke this is being treated as, I can't forgive his shitty parents! Especially his damn mom! Arthur's dad might be insane but his mom intentially left him behind because it was dangerous but had more kids anyway! Someone PLEASE punch the McShit out of them!!