Can a POA sister restrict the family from visiting dying mother in my sister’s home?

Is Mom coherant. If so, she can make her own decisions. The hospital likes to work with only 1 person. Being POA for medical she can choose not to tell you. She is suppose to be abiding by Moms wishes.

Maybe sister doesn't want people traipsing thru her house. Actually, if she is doing the caring she should welcome the company. Maybe ask why you all can't visit more often. Tell her by being there you all could be of help. She doesn't have to entertain you. Tell her to get out for a while. Shop, run errands, take a nap. That's what family is for, to help share the burden. A POA does not mean she is in command. She is suppose to think what Mom would want and do it.

Yes she can.   I am sorry about your mom and the situation.

 BTW, did you guys help out POA sister at all?

You say you do not have information, but it appears that you do understand that your Sis is and has been caring for your dying Mom, and you understand that she is dying. There are six of you, and your Sis is dealing with the care of a dying person. In my opinion her requirements are entirely reasonable. You will undoubtedly do a lot better by offering meals, support, housekeeping, casseroles, love, and asking "What can we do for YOU". Animosity will get you shut out, imho. It is up to your the approach you take to your caregiver sister; the consequences will you yours as well. I am sorry you are facing this sad loss.