Are there any witnesses who saw the three of them camping

The sister, Cassie, and her kids supposedly visited the brother and parents on Sep 6th at the Fort De Soto Campground but only for a few hours and although she seems pretty honest to me, who knows how reliable that is.

They had, what, 2 weeks to line something up?

Yep, Sep 1 looks like when Brian & family started planning his escape. He returned home, few days later he gets a new phone. Brian wasn't named a person of interest until the 15th.

I think they either drove him to Texas near the Mexican border or put him on a small boat headed to the Caribbean or Mexico. Either way, I think his father paid to have him smuggled out of the country,

Could be, whatever the means his family definitely conspired to get him away. I just checked, they're worth $5 million. Explains a lot.