It is trivial to eavesdrop on a wireless connection, especially if you are a part of that wireless network. From the bits you have shared in this thread, it sounds that he is either a) sniffing the wireless network (which would reveal any unencrypted traffic from any wireless computer on the network) or b) setting up pcs to use a proxy service he oversees.

Have a talk with him. Ask him about it. This almost sounds like "My bf/gf is spying on me - how do I confront them?" The chime x3 is enough to raise eyebrows, so go get answers. You may find out that he is doing some really cool stuff - just with the family as his test subjects.

The mail downloaded to your pc is a possible oversight. Some mail programs do such actions and rarely inform the user, so this may have simply been a mistake and not malicious.