I need advice regarding a living situation involving a person with dementia.

I have a new question, which is essential to doing the best for a person with dementia. A very much loved grandaughter was offered a summer job in the area my Mother lives in. My mother was delighted and excited to invite my daughter to stay with her. She has extra bedrooms and baths. A rotation of companions is there pretty much 24/7. The sister in charge is forbiding it. Wouldn't the best thing to be to try a couple of weeks and see if my Mom is happy and comforted by having my daughter there? In this case, my mom has wonderful long term memory and is still self sufficient in her own care.