No, it would not be legal in Pennsylvania to sell cups and then give away alcohol so that people can fill their newly-purchased cups.
You must be licensed to sell alcohol in any way, shape or form. Any unlicensed establishment ("speakeasy") is subject to seizure of all alcohol on the premises and criminal proceedings.
Unlicensed and illegal sales of alcohol will result in a fine of $2.00 per fluid ounce of malt or brewed beverages, a fine of $4.00 per fluid ounce of wine or liquor, and all alcohol will be confiscated, and a lien can be placed on the premises where the alcohol was served.
It is also illegal in Pennsylvania to have a ticket price or entrance fee to an event that includes "free" alcohol in the ticket or fee price, if the venue is not licensed to sell alcohol. I mention this, just in case that was your next idea.
All of this is covered under the Pennsylvania Liquor Code, Title 47, Section 491.1, 492.2 and 492.3.