Is a sponsor free to withdraw sponsorship of an adult child/stepchild and have no consequences or binding obligations?

Sorry to learn of all the difficulties and problems you have encountered with your mother and step-father. Regarding immigration-related issues, those who signed and provided Affidavits of Support may withdraw them before an application is approved and immigrant status is granted, but they cannot withdraw or cancel them after the granting of immigration benefits. Although not entirely clear to me from your post, it sounds like you already have become a Permanent Resident through a family-based application, and if that is so, you will not need to worry about withdrawal/cancellation of the Affidavit of Support. The Affidavit of Support primarily is intended to safeguard the government from having to supply needs-based public assistance to you. If you were to qualify for and receive such assistance, the government then could seek reimbursement from your mother and step-father who provided Affidavits of Support. On a separate note, I recommend that you get advice concerning the threats and actual abuse and violence you mentioned. You might speak to a school counselor, the police, the Department of Family and Children Services or others to find out what steps should be considered so that you can live in safe conditions. That might include, for example, obtaining family counseling for all members of your family, but that also might include, for example, seeking assistance from a shelter for victims of domestic violence, seeking police intervention, etc. Depending upon how threatening your situation is, you should not delay seeking this type of help.

The form I-864 is a legally binding agreement that can be enforced in Federal Court at a minimum. However, you will likely need help to do it. You may want to contact a not for profit immigration organization in Chicago, like the National Immigration Justice Center for help. Good luck.