Look, not2cleverRed made a good point earlier and I'm going to expand on it: why couldn't your sister-in-law simply take everything to the post office, shove it all into flat-rate boxes, pay for the postage, and send it to you? If money is an issue, she could get the boxes, which are free, pack everything up at home, and have you send shipping labels. If money is not an issue, ask yourself why she doesn't ship the items, especially since this kind of bad behavior is a pattern based on your posts. Is your wife's sister scared of her husband? Or are they both untrustworthy?
Rather than getting mad because you're not getting the advice you want to hear, you, your wife, and your wife's parents need to figure out how to negate any damage these shenanigans can cause the family in the future, and how to do it with minimal drama. I can't tell you what to do but based on my experience with my late parents, they would have simply stopped sending gifts to the adults on that side of the family for birthdays and Christmas, and only the grandchildren would have received presents. Any medical or financial POA's would have gone to the trustworthy children, and their Wills would have been set up so that the untrustworthy children were not listed as executors. If my parents had suspected spousal abuse or if a child was bad with money, a felon, an alcoholic or drug abuser, etc. then the inheritance would have been left as a trust to be doled out over time rather than a lump sum.
Hopefully I've given you something to think about other than getting the cops and courts involved in this mess.
Good luck.