I would tell SIL that from now on, she needs to leave a specific message as to why she is calling, if not, you will not be returning said call.
If she is at the county med, then you will know. If she just wants something, you will know and can prepare your response.
Your sister is 60. Your husband is not responsible for her. As long as he enables her, she will not learn to "grow" up.
There are some things I would still do, but I would not be at her beck and call and I would not support her financially.
As for grocery shopping, I would tell her this is when I am going, do you want to go along? And that will be the only time you are going that week, so she needs to get what she wants then. If she calls you later and needs to go, tell her you already went. If she needs to go, she needs to find another way there or wait until you go again.