Hi WANTMOMBACK. This is so sad. There may be a way of handling it nicely so you and your sister don't need to cave in again.
Go. Make good use of your time there.
Ask to speak with an elder about how your mother compels you — an non-interested adult — to attend or be cut off. Ask him to show you where Jesus commanded this behavior, especially at his Memorial. He'll have to admit, you're technically not even of the "other sheep!" Ask him to explain to you (in front of mom) why she should shun you, and where this is taught in the scriptures, and how it applies in your case. You are no different than a non-interested householder, or a study who just doesn't feel like attending, and should be afforded the same respect.
Let her push and push and push. Let her do it in front of an elder, and see if he doesn't kindly "adjust" her thinking for you. The fact is, she is giving a very, very bad witness and this reflects on the whole congregation. The elders bear a responsibility in this matter just as much as your mother does.
Make a JW parent accountable for his/her selfish unChristian unscriptural actions, and s/he will usually back down from threats and emotional blackmail. (I don't mean to attack your mom, I am pointing out the nature of her actions as I interpret them from your description.) Use their own bible to do so, and involve the elders if you must.
Good luck!
~Sue