So, he visited them abroad and brought the items back with him, rather than them paying for international shipping.
This is your wife's family. Is the issue that your BIL doesn't want to foot the cost of shipping to your wife, or is he just being difficult?
If it's the cost of shipping that is the issue, the solution is obvious: you and your wife pay for shipping the gifts.
If your BIL is just being difficult, then I really see no easy legal option for your wife. Your in-laws are the ones to deal with the issue, not your wife. I suggest that rather than feed into his silliness, just rise above this and be non dramatic. YOU, in particular, be nothing but pleasant to all of your wife's relatives, including the problem BIL. Your wife needs to work out whatever her problem is with her brother - don't contribute to the problem. It would be worthwhile, though, the mention to her relatives that the gifts never made their way to her, that her brother was unable to send them, but that she appreciates the generous gesture. She should not lie to her relatives and say that the gifts were damaged en route, or some such thing to cover for her brother - she should leave it vague and open ended enough that her relatives ask HIM what is up - since apparently only he knows.
As an aside: do you suspect he is selling the gifts or just keeping them?