What is to be done concerning a married sister who committed adultery, became pregnant, and then aborted, knowing that a previous attempt to deter her was unsuccessful?

What you did by gradually advising her was correct. Since she did not deter, it is obligatory to inform someone who can restrain her, such as your father, brother, or paternal uncle, and to threaten her with informing her husband. Show her the fatwas numbered: 43879, 47988, 51140, 34204, perhaps she will repent. Do not mind her anger, for what you are doing is the truth. The Prophet, peace and blessings be upon him, said: "Whoever among you sees an evil, let him change it with his hand; if he is unable, then with his tongue; if he is unable, then with his heart, and that is the weakest of faith."