By disassociating yourself from the org you are following their policies - the rules of engagement will automatically change. You are allowing the organization to separate your family from you. Your parents & sister are fully involved / invested in this belief system. That is their choice. You've made your position clear. Disassociation will prove in their minds that you are 'mentally diseased' and deserving of shunning and final destruction.
As JW's we're trained to blame everything on the 'outside' for all of our problems. Your mother feels abandoned by you because the religion demands that the faithful choose it over family. By blaming others for your position, you parents can be free from guilt because of your personal decision. And, you ( as their daughter ) would never behave like this! It must be the fault of your husband and his family....
They're just scared of the 'reproach' and consequences of your decision. If you want to communicate with your parents for the rest of their lives, just carry on without making a public statement.