"What the hell were you thinking by telling him about Marisa and my kids?" Giovanni attacked his father the moment they came back home, after meeting up with the detective who will be here at the family house very soon.
The old man's quarters have been sealed off for investigation purposes, and he didn't care about anything that has to do with his grandfather. If his father or anyone in the family wants to go ahead to find who murdered him, that's none of his business.
But what he won't take is anyone going ahead to talk about him and Marissa, his father has gone ahead to tell that detective about Marissa and the kids. He was pissed at his father for what he had done.
He's not a fool. When Marissa went over to live with that detective, he wasn't blind not to notice that the detective had something for Marissa. No, it wasn't just after she stayed at his place.