I am very sorry to learn of this situation, and my sincere condolences for the loss of your wife.
Unfortunately, the very best course of action to get resolution here is to make a police report - you were robbed.
You can make an insurance claim for the loss (even without a police report) but without the report it will make it harder to prove the loss (the carrier is more likely to approach this as it being a "gift") - but rather than simply disregard the potential claim, you can certainly file one anyway.
You may want to start by simply letting the individual know that you are aware that they took your property, and that you would like it back, if you receive all of it back you are willing to forego legal action against them. I understand that this is probably not something you want to consider in the wake of losing your spouse, and in dealing with family, but you were victimized by someone (apparently a couple of individuals) who showed no respect for you in this time of loss, and at a minimum you should have all of your items returned and accounted for.
Ultimately how you handle the matter is up to you, but the above is the outline for what you have to work with.
Again, I am very sorry to learn of this situation.