Thanks, all. I will look into the drink cup holder. After she "contaminated" both soda bottles, I bought another bottle and shoved it between my seat and the door. It felt weird, but, dang it all, I wanted to drink ALL my Diet Coke.
Someone explained to me one time that, for most heavy drug users, her behavioral and emotional age is that of when she started using, in her case, 12. So it makes it tricky to call her out, because we never know if she'll react as an adult (62) or as an over-emotional preteen. She acknowledges that she has made bad life choices and has made a conscious effort to overcome her long-term addiction.
My DH's solution is to stop drinking in the car and/or stop going to restaurants with her. He's not very attached to her, but is her only close relative. It falls to me to keep up the communication and, except for this issue, hasn't been a problem for me (as long as she stays straight). I'm going to see her next week and I'll try the above suggestions. I'll update post-visit.