Now the relationship between the two is confirmed in the lines:
And I have much to thank her for. This son she bore – first blood to her.
Despite the speaker’s assertion that she ought to feel gratitude to her lover’s mother, instead she is acutely aware that a blessing for their romance will not be gained easily.
The dashes in these lines isolate the words to ensure the reader/listener finally understands the root of the conflict between the two women. The nature of their relationship is finally and unambiguously revealed.
Repetition in the line
never, never can escape scot free/the sour potluck of family really reinforces her belief that her lover will not be relinquished to her without a fight.
Her word choice in describing the family we are born into as a 'potluck', a lottery in essence that in this case is 'sour' reveals the arbitrary nature of families and their makeup.
The tone changes in the final two lines to a wry acknowledgement -
And oh how close/this family that furnishes my rival’s place.
The suggestion implied is that this is a family that is too close, that is suffocating and claustrophobic.
The speaker is very aware of her position as an outsider, of not belonging and of the inherent threat she poses to this matriarchal, controlling figure.