my theory
I had the following theory. The cobra does exist, because the other family members saw it too.
The younger brother gives us a hint as to where the snake may have come from, that it got here through Air India. If we want to dismiss the fact that the cobra came from the potion, this may have been completely plausible. Perhaps it slipped into Chand's luggage before she left.
In many times of abuse, victims create safe fantasy zones for themselves (similar to Pan's Labyrinth) where they can escape to to cope with their hurt. I believe that Chand used the real presence of the cobra and created a side line fantasy to live in as a way to find happiness, both in connection to her mother (as she was the one who told the cobra story) and to live the initial hopes she must have had when she first moved to Canada (of having a good husband who loves her).
this is my theory, I think it's open to interpretation. Anyone else?
Eagles may soar but weasels never get sucked into jet engines