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.