She hasn't acknowledged him as a person at all. That's the point. There is so far no evidence that Elliot and MR are seperate entities. On the other hand everything makes more sense if he's in Elliot's head and we had a scene this ep where MR's face flashes over Elliot's. Doesn't get much more explicit.How do you imagine Darlene should have acknowledged mr Robot as their father if he was real? Constantly calling him "dad" or what?
How come we've never seen Angela mention Elliot's sister to him before? Angela and Darlene are friends apparently? You can imagine it would come up.
For your second question, the only connection between Elliot and Darlene seems to be their involvement in fsociety and their project to bring down the biggest corporation, that Angela, by extension, worked for. Not exactly smalltalk material.