The law is clear.

The obligations of the household member who signed the I-864A with the sponsor terminate when the obligations of the sponsor under I-864 terminate.

If you are the sponsor, your soon to be ex-wife will still be bound by the I-864A until your obligations are discharged.

Your obligatiosn end when either the sponsored immigrant becomes a U.S. citizen or he/she has 40 qualifying quarters of work in the United States.

Divorce has no implication on the validity of I-864A. Think about it this way, if a divorce can get someone out of I-864A, a lot of people would do it to protect their assets.

So whoever signed the I-864A as a household member will remain responsible until the obligations of the sponsor end, no matter what.