Your BIL is seriously overloaded.
I had a Chevy Venture ninnivan with a 3.4L. Max tow capacity is 3500 lbs. I pulled a 2500 lb popup with it. I wouldn't add 1500+ lbs to the rig.
His trailer is over 3500# before he attaches the ninnivan to it or adds the "stuff".
He will likely kill the tranny very early in the Venture's life. The rear suspension can be heavily damaged. There is a host of other things that could die on the van.
Now, he will say, "I only go 5km..." During that distance the trailer will be in control. Stopping could become an unpleasant experiance if he has an emergency...say like a kid suddenly appearing chasing a ball....
A blow out on van or trailer would prove to be interesting. Does he use brakes on the trailer and a brake controller?
Does the van have a tow package or the required towing add-ons? Like tranny cooler, oil cooler, large radiator. Heat kills engines and trannys fastest.
Technically, a 8 HP lawn mower could pull the trailer, if geared correctly. I wouldn't recommend that and I don't think your BIL has a good situation for towing.
Urge him to get a proper tow vehicle for his trailer.
I'm the lucky one, my wife married an idiot.