This is a 3yr old home and this summer the plumbing gremlins hit. Seems when we use the kitchen sink or the master bath sink it makes either the master or the half bath toilet put more water into the bowl...Any guesses??

If the level in the bowl is rising without more water coming through the float valve, then it is very likely a venting issue; the air is trying to find a way out to equalize pressure after being displaced from the water flow.

If rapid changes in water pressure is causing the float valve to release more water, then the problem can usually be addressed by adjusting or replacing it.