Unfortunatly there is no middle ground with the org, if you choose to be inactive that is the same as saying you don't want to be a JW, which is the same as disassocation.
The real world isn't so black-and-whiite, but they are.
Also remember their definition of an apostate, is as simple as someone who believes differently or leaves. You don'thave to speak out or write books, private thoughts are held against you.
You may want to spend a few emails just saying Hello, talk about your day. Just try to establish conversation that is not about anythign JW. Hopefully your sister won't interfer. If you mothin has made a choice you probabky just want to be friendly for a while and not give you sister any reason to block your email.
I wish you the best and congatulations and taking control of your own life.