Answer:This line "My Mother Really Knew" is in itself a complete justification of how his Mother must be going through all the life-threatening issues with his father being alive or even after he was dead.
Explanation: In the second stanza, she has explained all her hatred and devastating feeling of hers for the decade of kissing him every night before going to bed. even if it's a peaceful day or a night with big arguments or his mother and brothers acting completely blind by not even makin him realise what the issues he is trying to create every single time.
The title of the poem is in itself is a huge statement of what she must have gone through after knowing all the deeds he has done so far. for which he didn't get punished.