I don't know ... it's confusing, because in another article, Wikipedia referred to Princess Victoria of Saxe-Coburg and Gotha (Kohary) as the *mother-in-law* of Duchess Sophie Charlotte in Bavaria, despite the fact that she died young (in 1857, aged 35) -- before any of her children married. So I see a complication, here, since Winifred Wagner is referred to simply as the wife of Siegfried -- not daughter-in-law of Richard (although she most certainly was Cosima's daughter-in-law).
Wikipedia is alright but we mustnít be overly reliant on it. If a term is unfamiliar or perhaps uncommon it isnít necessarily incorrect.
Another complication is if the spouse dies, especially if the person remarries. Interestingly enough, the composer Giuseppe Verdi continued referring to Antonio Barezzi (whose daughter Margherita he had been briefly married to) as his *father-in-law*, even after his wife's death. He remarried many years later, with Giuseppe Strepponi, when Barezzi was still living. One presumes that he no longer referred to the older man thus ...
In various times, cultures, etc, a widowhood locked relationships (and sometimes titles) in despite remarriage. Divorce or annulment might truly break such things.
In fact, I've heard of the expression "former mother-in-law", applicable to persons who have been widowed or divorced.
My last living (and male-line) great-grandmother died when I was 25 years old. My paternal grandmother, the widow of the grandfather I mentioned earlier in this thread, still referred to my great-grandmother as her mother-in-law... in spite of the fact that she has long been married to my step-grandfather. My grandmother is still the widow of my grandfather in addition to being the wife of my step-grandfather.