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

Short answer, yes, if she is the medical POA. She is charged with making decisions intended in the best physical/medical interests of your mom.

why can't one person visit in the morning and one in the evening for the first day, then do the same for the next 2 days and hopefully she will be around for all of you to say your goodbyes.  I don't know if she can do that or not, but I pray that she will at least let you visit even if for 10 minutes.  wishing you luck and peace

No one, including doctors and caregivers, can pinpoint the time and date of death.

Given Covid, I can see why your sister may not want a deluge of family.

Regardless of Covid,
If your sister has been a primary caregiver, you should respect her. She knows most about your mom’s wishes and daily routine. She is probably trying to keep the environment as calm as possible for your mother.

You should also think of your sister’s feelings. Her nerves are Certainly rattled.

Having been around multiple people at end of life, I can also say I would not want crowds of people descending on me in my last days. Commotion, noise, chaos, crying, and even apathy would probably all be displayed.

If you have all been visiting regularly this should not be an problem.