One of the challenges family members face when they are dealing with a loved one with memory loss is the sometimes-daily repetition of questions, the answers to which have been given repeatedly.
The questions about dead relatives is a common one, and for your mother-in-law, each time she asks the question about dead relatives, it's the first time she's ever asked. However, for you it's almost like living inside reruns of the Bill Murray movie, "Groundhog Day" in which no matter what Bill Murray did, every day was an exact repeat of the previous day, with the same questions, the same answers, and the same situation as if the previous day had never existed. That for many victims of Alzheimer's is an active part of their disease progression.
Your mother-in-law has no memory of the death of a family member, and when the thought occurs to her that she hasn't seen her mother, brother or spouse lately, she has no recall of their death, and for her is simply asking a logical question "“ over and over and over. She is no longer capable of processing and retaining the information or the response you are providing because she is, at least in her mind, living in a moment in time when that now deceased person was still alive.
It's definitely not necessary to take her to the cemetery to prove to her that these relatives are indeed deceased, because within hours or days, she will not recall having visited their graves, and will ask where her dead relatives are today.
There is no solution to your mother-in-law's memory loss issues, but you can turn her forgetfulness into an advantage for you by responding to the question, "Where's my husband (brother, mother) by saying, "He's busy right now, but he said he would stop by to visit later." Your mother-in-law will not remember your response, nor see it as a lie, she will simply forget, and probably ask again later.
This is a technique of redirecting your mother-in-law, and using her memory loss to your advantage. However, you will not eliminate the repetition of those questions because that is part of the disease.