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.
The people who told you about SIL's emotional age are right on. The drugs basically prevent that person from emotionally maturing, so you are dealing with a preteen. That said, this is not your problem. SIL's emotional condition is a result of her bad choices. The resentment that she and your MIL feel toward you because you "stole" DH and DH was successful, not your problem. Their resentment was a choice that they made.
Part of the problem with dealing with people with substance abuse issues is that enabling family members have prevented them from feeling the impact of their choices. It's very difficult for a loving parent to let a child hit rock bottom, so they do everything they can to cushion that fall. Excuses are made. Boundaries are crossed. The people around them become miserable, but the drug user just wanders along in their addiction. Sober or not, this is the operating system SIL is used to.
That said, none of this is your problem. Your husband is not too keen on spending time with or contacting her. Why are you furthering this relationship
? Is it guilt? Fear of leaving SIL alone? SIL's choices have left her alone. Not you. Take a cue from your husband and withdraw. Have you considered that spending time with SIL may be making your husband uncomfortable? Maybe he would prefer it if you spent less time with her.
If you want to continue spending time with SIL, you have to tell her, "Don't drink from my bottle/glass. Drink from yours." I agree that this has something to do with both a weird family thing and domination. She wants to provoke into saying something to her. She wants your attention, and this may be the only way she knows how to get it.
If she acts offended, so be it. If she continues to do this after you tell her no, stop going out with her!
If SIL can't respect something as simple as "don't drink my drinks," imagine what other boundaries she could roll right over.
It is not your job to provide SIL with a family outlet. It's not your job to help her grow up.