I have done several oil changes on my 07 T/C ,also done the service for other friends and I'm not sure why but it seems these vans are highly liked in my area. To get to the point, every van no matter if the 3.3 or the 3.8 they all had a 13mm head bolt. My sister inlaw has an 07 base model w/ 3.3 but has a 15 mm bolt, just seemed odd.

 

Perhaps your sister inlaws's car sump plug was over tightened and stripped threads in sump resulting in it being replaced by a bigger plug from another car after Drilling & Taping. Heli-coil would be better answer to using same plug but depends on what's at hand.
Come to think of it does America still use Whitworth/anc/etc threads like we used to have or are they compliant with Europe on metric like Blighty.