"You killed my moms sister."

lmao.

I haven't even seen it but that quote you gave sounds pure Spielberg.

Yeah, you can say that. But I get the sense he said it the way he did in order to convey and emphasize how deeply personal this was for him (dramatic effect). That said, in spite of what happened to her it’s not unlikely Wade may have still resented the fact that his aunt would bring in a bunch of lowlife creeps to live with them. Maybe he hesitated to call her his aunt for that reason even though he did care about her.

The former is the likelier reason, albeit it could be both.

I just heard "Hello, My name is Inigo Montoya, you killed my father, prepare to die"

It did sound funny but I guess it was to emphasize that he was not in the negotiating mood. We all have many aunts, but to have orphan Wade's only close blood relative left (I'm assuming) murdered like that means all bets are off. As for that guy the aunt was seeing....eh, Wade misses him less.