To what extent can we alleviate the burden from my deceased sister, who uttered the Shahadah and then spoke other words, and whose prayers were intermittent?

The severity of your sister's death, God willing, is an expiation for her sins, for everything that befalls a Muslim, Allah expiates some of his evil deeds thereby. Her death in a car accident, God willing, makes her one of the martyrs of the Afterlife. The fact that her last words were not the Shahada does not imply a bad ending for her. I advise you to supplicate, seek forgiveness, and give charity on her behalf.