Is it the POA's responsibility to help do my aunt's (86) taxes?

Who did her taxes last year? Start with that return to understand how complex the return will be. If it is a long, complex affair, I would see a tax consultant and I would definitely need written, witnessed POA for doing the tax stuff.

If your Aunt doesn't have any type of financial Power of Attorney, and she still has her wits about her, she should be able to choose whomever she wants to help her with her taxes. It's up to you whether you want to do this job or not. You could recommend an Accountant or CPA to start doing her taxes, that way if your Aunt has memory issues down the road, the Accountant/CPA knows what to look for in the way of paperwork.