The first-born said "my father"

I stumbled upon this verse thinking that they have a different father but the same mother.

And on the next day, the first-born said to the younger, "Behold, I lay last night with my father; let us make him drink wine tonight also; then you go in and lie with him, that we may preserve offspring through our father." -- Gen 19:34 (RSV-CE)

But reading into the whole context, I think the girls have the same father -- Lot.