Would her child be able to obtain power of attorney to make health decisions on her behalf?

No. If your aunt is in a coma, she would not have legal capacity to appoint someone to be her agent through a power of attorney. If at some time in the past when she did have capacity and she appointed someone to be her agent through a power of attorney, then the agent's powers may endure after her incapacity. This is a sad circumstance but it illustrates the necessity of advance planning, e.g., wills, healthcare powers of attorney, medical directives, living wills, beneficiary designations, etc., for matters such as this.

The simple answer is "no" - she does not have capacity to appoint an agent to make health care decision, and these documents should've been prepared in advance. The more complex answer is that if the husband is not acting appropriately, you may be able to file suit and have the court appoint yourself - or someone else - as conservator and/or guardian. Simply withholding information from you is likely insufficient grounds for the appointment of a guardian, and this is a fairly expensive and dirty process, but it can be done in some instances. In any event, you should contact a local estates attorney to discuss the details and see if any other options are available.