The 15- and 18-year-old sisters allegedly threw a party without permission while their parents were out of town, a McHenry County Crime Stoppers news release said.
The theft allegedly occurred February 11 in the 800-block of Florence Street.
About 25 teens attended the party, the release said, and afterward, about $20,000 in cash and about 100 Hydrocodone pills were found to be missing. Police say they suspect someone at the party carried out the theft.
Anyone with information is asked to call the McHenry County Sheriff's Office at (815) 338-2144. Callers can remain anonymous and may be eligible for a reward up to $1,000.