Fear of Incest?

Building on Mir Sanaullah's post:

Ehsaan Manzil comes to depict a very different kind of parda, which includes daughters’ parda from father and brother. The daughters of Shareef  Muslim family are kept in seclusion from their “mehraams” and the idea seems pretty odd. I feel, the parents’ fear of keeping their daughters pure has crossed extreme limits, which has led to parda from brother and father. Fear of incest could be one way of looking at this. What other reason could it be to not allow one to communicate with their OWN father and brother? They aren’t even stepfather or stepbrother.  Had that been the case the parda would have made more sense.