My sister-in-law is causing problems in our marriage of 42 years. She is twice a widow most recently 4 years a

She is 10 years older than my husband (who has always had a backbone problem). I was diagnosed with MS 5 years ago, I don't have the energy to squabble with her. She keeps calling my husband all the time for this and that and asking him to go with her to different functions (most recently a dinner theater-which they attended while I stayed home.) She knows I am unable to attend many of these functions so she basically uses him as her escort and designated driver--she is an alcoholic and she has alienated so many people including her own grown children. This has been going on for several years. My husband blows up at me if I mention that it is upsetting to me. I don't know what to do.