Where to find help with mom's credit card balance payments and Medicaid spend down?

Retain the services of an expert elder law attorney who is well versed on Medicaid. They will go over EVERYTHING with a fine tooth comb. This attorney will be expensive but worth every penny in my opinion.

Both mom and sis are on the mortgage. The hard thing with the credit cards is since they lived under the same roof, sometimes sis would charge things on the credit card and give mom cash back when she got paid; sometimes mom bought things on the credit card for my niece, etc; sometimes sis would charge something on mom's credit card and pay another bill for mom, etc. Finances were all mixed up. Should sis go through statements now and just try to address some things that she remembered or submit the application and wait to be asked? Thanks

Medicaid will want to see all the credit card statements for the last five years. They are not so concerned with sis paying the bill, but they will question whether sis benefitted from the purchases.
It is very important to know whose name is on the title of the house. If it was only sis, there is a big problem there, Medicaid would call that a gift and impose a penalty. Get all the check images for the last 5 years.

What jumps out for me is that your Mom was paying your sister's mortgage.... is that correct? Or is the house Mom's? If the house belongs to your sister, then Medicaid might view that as a "gift".