It seems weird all of your family have same symptoms. It all points to something you are all eating, or maybe drinking.
You might want to read my posts they are very similiar and its really frustrating when the doctors cant get to the bottom of things.
I contracted a camplylobacter bug twice in last two years and i believe the aftermath of that is why my stomach is dreadful these days. that can come from contaminated dairy and or meat.
Was there a time when you all eat something like a joint of meat of a BBQ? I would ask the Doctor to test you for that too. Although amplyo gave me really really bad diarroreah and you dont seem to have that, unless it doesnt always come alongside diarroreah. Check it out and good luck. I know how you feel.