Thank you. The short answer to your question is, no, she cannot unless she is his personal representative (executor) of the estate. There was no will so she cannot be.
Even if she had power of (which I accept that she did not) it ceases on death.
What you have been told is correct, it’s a civil matter although I don’t actually know how your sister is trying to ban you from the funeral because short of having a fight, you can simply turn up.
I am not certain whether that answers the question for you or not but I am happy to answer any specific points arising from this.
Can I clarify anything else for you?
Please note that my answer is based strictly on the information that you have given me. If you have not given me all the information, then my answer may be incomplete or wrong.
I am happy to answer any specific points arising from this.
If you still need any points clarifying, I will be happy to reply because the thread does not close.