We are on a sewer system. We have a two story home with two full baths upstairs, and one half bath,laundry room and the kitchen downstairs. Water pours out of the toilet in the half bath downstairs every time we flush the toilets upstairs,wash clothes and run any water in the showers or faucets throughout the house. My husband got on top of the house and checked the vent pipe for a clog and didn't see one. I greatly appreciate any suggestions you might have.

My husband got on top of the house and checked the vent pipe for a clog and didn't see one.
Hubby had the right idea but you can't tell just by looking down the vent. You have a clog in the drainage downstream from the toilet that overflows. The solution would be to rent a sewer machine, (see image) and snake from the lavatory roof vent of the half bath. This vent can be found above the location of the half bath. If you omly see one roof vent for the entire house let me know right away. Put out enough cable to reach the base and 16 feet more. Good luck and thank you for rating my answer. Tom