Can my aunt get power of attorney for my grandmother's finances without her other siblings' authorization?

The power to name your aunt as agent in a power of attorney for finances would come from your grandmother—and she is free to name whomever she wants in this role, as long as she is mentally competent to do so. No one else would need to be consulted or to authorize the arrangement.

If your grandmother is not mentally competent to finalize this paperwork, the alternative would be to secure a legal guardianship or conservatorship for her--a procedure that takes more time and effort. If this might be an option for your aunt, she should consult the local probate court for more information. Before a guardianship or conservatorship would be granted, however, the court would notify her siblings about the procedure and give them the opportunity to contest it.

For more on powers of attorney for finances, see Power of Attorney for Finances: A Beginner's Guide. For more on guardianships and conservatorships, see Conservatorship and Adult Guardianship: A Beginner's Guide.