What is the immigration status of your step-children? If they are green card holders, it is too late for you to "withdraw sponsorship" of them.
I'm sorry to hear about your separation. I agree with Mr. Shusterman. If they are greencard holders, it is too late to withdraw a petition and your mother can be held liable for any public assistance they ever obtain (re-read Form I-864 contractual obligations), albeit unlikely. If they are in the process of applying for adjustment of status, the petitioner can withdraw/revoke the petition, and the co-sponsor presumably can withdraw their signed form I-864. I hope this helps.