How to read data from Arduino with Raspberry Pi via I2C?
You might want to graduate to the pigpio library for the Pi side of this.  

Here is where you can find examples of how it works.

here is a fun one that is like Wireshark - will show you the I2C traffic:

https://github.com/joan2937/pigpio/tree/master/EXAMPLES/C/I2C_SNIFFER

Here is the FAQ about I2C