We have had an in-law move in with my Mom. She gives us her consent but doesn't remember. How do we handle her anger and resentment?

My Mom has lived alone for 21 years since my Dad passed. She has early stages of dementia and her neurologist has indicated that not now, but soon, she should consider moving in with someone of having someone live with her. She has her own home and we would like to keep her there. She is very independent and doesn't believe that she needs any assistance. Right now my brother and sister-in-law manage the meds because Mom gets confused about the day and was over medicating herself. I live about 1.5 hours away. My sister-in-law's Aunt just happened to need a place to stay and we have known her for more than 25 years. My brother and I approached Mom about having her move in. We were honest about her needing a place to stay but also that she could help with meds or anything else needed. Mom was a little resistant initially and doesn't remember any issues with medication. My brother and I both confirmed that there were issues and she agreed to allow her to move in. Mom was concerned about having to "take care" of her new roommate. This is not an issue as she has her own car and her family lives close by.

My problem is Mom calls me everyday and wants to know if we orchestrated this plan to have someone move in without her consent. I try to assure her that we did not orchestrate a plan that we definitely consulted her and allowed her to make the decision, but she is convinced that we did. She also feels like her new roommate is a house guest and that she must cook and prepare meals for her. She has even asked if she is going to stay forever. I can feel Mom's frustration and anger. This is new ground for all of us who know and love Mom. I want her to be comfortable and happy. And I want to understand the world from her perspective and provide her with all that she needs to be healthy and happy. We don't know where this disease is leading us, but we want to do right by Mom. Mom is a retired teacher and my Dad was retired military. So, insurance coverage is not an issue. But no matter what we do, if she doesn't understand we are doing it because we love her and want the best for her, it leaves us feeling sad. I wish I could understand how her mind is processing things and communicate with her on that level. Please advise us.