Rain sensor + transistor
I believe this should be able to work, and I will provide the calculation for selecting the correct base resistor to ensure the transistor is turned fully on.

You state the sensor draws 15 mA when on, so let's be conservative and aim for a collector current of 20 mA. According to the data sheet of the transistor it has a current gain of at least 100, therefore the base current Ib must be:

$$I_b=\frac{I_c}{h_{FE}}=\frac{20mA}{100}=200uA$$

The resistor is then chosen to allow this amount of current to flow when 3.3V is applied by your GPIO. The base-emitter junctions typically drops 0.7V so there will be 2.6V across the base resistor. Using ohms law the appropriate restore can be calculated:

$$R_{base} = \frac{3.3V - 0.7V}{200uA}=13k\Omega$$

Round down to 10k ohm to be sure the transistor is fully on and you should be good to go!