Powerline adapter to connect 2 apartments in one building?
All legal arguments against doing this aside, I doubt that a powerline adapter is what you want here.

You will probably have better results with a "Wifi Range Extender" or a Wifi Repeater.  There are many suitable products on the market and some simple searching or a trip to a local electronics/computer products store should net you one.

In extreme cases, you might consider a directional-gain antenna that, in some cases, can extend the range from a few dozen meters to a km or more depending on terrain and conditions.