People diagnosed with dementia can still function in the early stages. Many can even drive and manage their own affairs.
I am proud of you for taking this step when she is still in the early stage of dementia.
It's a question of her lucidity (cognition) and ability to answer questions an elder law attorney would ask of her to ensure she is of "sound mind" and know what she is agreeing to.
Be sure to find an "elder law" attorney this specialty is versed in drafting documents for loved ones who have questionable ability to manage their own affairs.
Contact your local department on aging for a list of elder law attorneys in your area.