Dreaming of a deceased relative in a dream, if there is no special event related to it in the dream, or if the relative doesn't say anything particularly difficult to you, then such a dream is usually caused by missing the relative and has no special predictive significance. Because your mother-in-law is thrifty all her life, as a child, it is inevitable that she will feel that the elderly have not enjoyed enough happiness, so she will dream of shopping with her mother. Dreaming that your mother-in-law is lost means that you can't see her and need to find her, which reflects your consciousness of paying homage to her.
So it's normal to have such a dream, and it may also come from your concern about the strict investigation of burial. For this problem, you should try your best to keep it and let the old people rest in peace.