My sister is POA for our aging mother (in MI) but makes all decisions without informing or consulting with siblings. Is this legal and if so, where can we find this information?
johanna, your Mom choose your sister for a reason to represent her as her Power of Attorney. The sister can do whatever she feels is in the best interest of her [and your] Mom.
If you feel that your sister isn't acting in your Mom's best interest, then set up an appointment with an "Elder Law Attorney" and ask about Guardianship. Be aware that Guardianship is quite expensive to obtain.
https://www.agingcare.com/articles/power-of-attorney-explained-187545.htm
https://www.agingcare.com/articles/things-you-can-and-cant-do-with-poa-152673.htm
If you feel that your sister isn't acting in your Mom's best interest, then set up an appointment with an "Elder Law Attorney" and ask about Guardianship. Be aware that Guardianship is quite expensive to obtain.
https://www.agingcare.com/articles/how-to-get-guardianship-of-elderly-parents-140693.htm