Where's the proof they are 'scamming'?
Society has sunk to a new low, when people would willingly shop a family member over a few quid.
Find out all the facts first, then have a quiet word if it upsets you that much.
Imagine he consequences if you shop them, there's an investigation and they are exonerated/caught. They'll never trust you again, other family members will despise you, and it could drive an irreparable wedge between your wife and her sister and mother. Is it worth it?
This forum, eh? People willingly buy stuff from abroad, to save money and dodge tax, spaff over all sorts of filth on internet, drive above the NSL whenever they can get away with it, yet bang on as though they're as pure as the driven snow. Hypocrites.