This poem is about how the rise of the subconscious, and how sometimes being asleep and letting our inhibitions go might lead to the darkest of our subconscious thoughts surfacing. In my own reading, I don't see this poem as meaning that the mother and daughter are faking their love or kindness for each other when awake. I think it's more that when they sleep-walk, the darkest of their thoughts come out, the thoughts that appear every once in a while when they're angry or upset.
In the town where I was born lived a woman and her daughter, who
walked in their sleep.
One night, while silence enfolded the world, the woman and her
daughter, walking, yet asleep, met in their mist-veiled garden.
And the mother spoke, and she said: “At last, at last, my enemy!
You by whom my youth was destroyed—who have built up your life
upon the ruins of mine! Would I could kill you!”
And the daughter spoke, and she said: “O hateful woman, selfish
and old! Who stand between my freer self and me! Who would have
my life an echo of your own faded life! Would you were dead!”
At that moment a cock crew, and both women awoke. The mother said
gently, “Is that you, darling?” And the daughter answered gently,
“Yes, dear.”