Thank you for your question.
Whether it would be valid or not depends on how far along her disease had progressed and if she were still competent at the time she answered you. Since she is in the very early stages of the disease, its very likely that at least right now, she would have a good understanding.
However, simply marrying her would not give you automatic control over her pre-marital assets. Certainly if she added you to her bank accounts, for example, you would have access, but otherwise she could still keep those assets separate and apart.
A better idea would be (if she is agreeable) having her execute a Power of Attorney
which names you as her "agent" or "attorney in fact" to do things like manage her funds, pay her bills, etc. for her when she is not competent to do so. It would terminate upon her passing, and as her agent, and you would of course have a fiduciary duty
to act in her best interest for her as her attorney in fact.