How much are my sister and I responsible for parents once their assets are exhausted if they eventually need a NH?

pstegman is right, If none of the parents money has been squandered or given away or somehow just disappeared, you may be okay. You have to keep track of everything and their money is to be solely used for their care, that pretty much includes no more gift giving to anyone.

Medicaid does a 5 year look back on the parents assets to look at their finances and where everything has gone. If $100,000 has been squandered then they basically make you pay it back by not covering your parent for a period of time. If a nursing home in your area costs $10,000 a month, then they will not cover your parent for about 10 months, during which time you will have to find and pay for alternative care.

Get an attorney that can advise you with the laws in your state. With the changes in Obama care alot has changed

In May 2012, a Pennsylvania appeals court found a son liable for his mother's $93,000 nursing home bill under the state's filial responsibility law. Health Care & Retirement Corporation of America v. Pittas (Pa. Super. Ct., No. 536 EDA 2011, May 7, 2012.
Bear in mind the nursing home sued because Medicaid did not approve payment prior to the patients death. So keep your financial ducks in a row. If you never took a penny from them, all well and good. If during the five-year look back there is anything unaccounted for , not so good.