All she has to do is renounce sexual relations. Since the marriage is invalid, her sin is adultery if her real husband is still alive, otherwise fornication. If still sexually active, she’d be required to remove herself from the proximate occasion of sin as well and move out. If no longer sexually active, she’d be allowed to continue to live as brother and sister with her 8th ‘spouse’.
If her real husband is still alive, she might be required to restore ‘bed and board’ with him, though that’s not always necessary if there is a canonically licit reason for her separation (ie, abuse, adultery on her husband’s part).
There might also be the sin of scandal, since others might have been led into sin by her example.
What’s important to note is that neither the sacraments of penance nor of extreme unction are efficacious without a firm purpose of amendment.
Anyway, ‘last rites’ would consist of confession, anointing of the sick (extreme unction) and Holy Communion.
Thank you so very much for the clarification and lucid narrative.
“Firm Purpose Of Amendment” means to go forward with Love for God, and renounce all attachment to sin, or occasions of sin, correct?
PS: This renouncement should be motivated by a desire to never offend God again.