To establish a relationship between two people, it takes two people. Therefore you cannot be related to yourself.
To establish the relationship between you and your great-grandmother's sister, she is still your great-grand-aunt, and if at some point in time your grandfather married her, she is your grand-aunt as well.
One of my clients married a woman, and on their wedding, her widowed mom met his widowed dad, they fell in love and married also. That made his wife his step-sister, all of his sister-in-laws also his step-sisters, and his dad became his father-in-law, while his mom-in-law also became his step mom. Later he divorced his wife, but his dad and her mom did not, so that means after the divorce his ex-wife is still his step sister. :-)
