Would the questioner be considered disobedient to her mother due to the obsessive thoughts that plagued her regarding her mother's illness and her desire to marry, and would her discussing these obsessive thoughts with her sister be considered a sin?

What you mentioned is not disobedience to your mother; rather, it appears you were dutiful to her. Your shortcomings are forgiven, by the will of Allah, as long as you are keen on being dutiful to her, in accordance with His (the Almighty’s) saying: "Your Lord is most knowing of what is within yourselves. If you should be righteous, then indeed He is ever Forgiving to the often returning [to Him]." Rejoice in the blessings of this dutifulness, rid yourself of intrusive thoughts and their consequences, and there is no sin upon you for speaking with your sister. Seek help from Allah and through supplication to get rid of intrusive thoughts.