My ex step sister who I think got paid thru state of SC was a home care provider for my dad. How can I prove neglect now he's passed?

Yes, sorry to say but as beneficiary she is entitled to all the money in the policy. There is no law that says she needs to pay for a funeral or pay his debts. Its her money. But...most of us would have used the money to pay for the funeral.

I will assume that Dad had the policy for his funeral expenses. If sister did pay for the funeral with the policy, I would not reimburse her. She is entitled to what is left over, she is beneficiary.

If sister was being paid by the State to care for Dad and Aunt saw abuse, she could have called the state and reported the abuse. Seems to me Dad would have been better in LTC.

When it comes to grands and great-grands, your Dad owed them nothing so neither did your sister. In my Moms Will and in mine, grands are not mentioned and there is no trickle down. Meaning if my daughter dies before me, her portion of my estate does not go to her kids. It goes to my remaining daughter. If my girls want their children to inherit, that falls to them to have Wills.

I am sorry for your loss.  If she was the beneficiary, it is her money.  You do not have to chip in for funeral if you did not want to.  She may not have been a great caregiver, but did she do more than the others?   If so, that may be why she got the insurance money.

He has passed. Proving anything at this time would be impossible.

I think that ship has sailed.
I assume sister had POA. I’ll go out on a limb and say I’ll bet you and your sister don’t get along well.
She was his beneficiary & dad designated her as such & managed his day to day care in her home. How often did you see dad in the 6 months before he passed away and was he content and safe there? Do you have any documentation describing sister’s abuse? Was APS aware? These are only a smidgeon of questions you’ll be asked if you get a lawyer.
Choose your battles; do you really want to pursue a dispute with your sister about water under the bridge? ? You both just lost your dad. Stressful times.
What damages would you seek?