Being the named agent in your mother's power of attorney involves some specific duties and responsibilities"”but none of them involve becoming her fulltime caregiver.
Most documents are quite
specific in enumerating the agent's powers, so check the wording in the one you have and hold. Generally, agents must manage finances or medical care according to the principal's "best interests." That does not mean you must take her into your home at the risk of jeopardizing your marriage"”only that you must make the best possible decisions for her if she becomes unable to do so for herself.
But you are also wise to be thinking ahead. And reading perhaps a bit between the lines, you are also wise to be concerned about defining your future role in your mother's life and care. Much of that has nothing to do with the aforementioned power of attorney. It's a practical matter, reinforced by habit and reality. Many families come equipped with one soul who fits your description: responsible, concerned, head firmly on shoulders. No tags back from the other siblings. So "the way it usually happens ”or often does, is that the responsible one insinuates herself or himself into the role of the parent's primary caregiver"”and the other siblings expect or allow that to happen.
So while it may feel a little callous to you, now is the time to draw your own boundaries. If it's not an option for you to move your mother under your roof, don't make it one. To the extent possible, begin discussing other options with your mother. If she's determined not to move into some type of assisted living arrangement, look into the costs and availability of other possible options for future help in your area: homecare services, adult daycare and day health care, meals and social and monitoring visits from volunteer and community organizations.
By sticking to your guns and being as clear as possible about what help might be needed, you may even find some help from an unexpected source: your siblings. Don't count on it, but know that it's a pleasant surprise in many such family situations.