actually no, he really does said "I don't know my mother" i had the same discussion with another fellow who thought dante said "about my mother". just listen closely.I don't think it's confirmed Dante doesn't know his mother. As Z218 pointed out we may have misinterpretated it. Because the demon says "**** your whore mother".
And Dante responds "My mother? I don't know my mother but if your calling me a son of a bitch then you wouldn't be the first one".
When he says "I don't know about my mother" : as in if she was a whore or not.
I don't think it means necessarily that he didn't know her. That's what i thought also but Z218 pointed out that i may be misinterpretating.