Referencing local libraries from higher dir levels
The problem with a relative path for an include is that it has to be relative to one of the predefined list of existing include directories. This includes:


Compiler include locations
The Arduino core files
The locations of other included libraries
The build folder for the sketch


What it does not include is the location of the sketch, so you cannot specify a location relative to the sketch itself.

Further, even if you could, the compiler wouldn't know what library it was supposed to be referencing so, while the header would be found, any source files associated with it would be ignored and linking would fail miserably.

Instead you will have to place the library in a standard library location, such as the libraries folder in your sketch book.