Irish Rose, I know how you must feel. I had a golden ring I had bought in Thailand many years ago and when some "friends" came over to visit their child was snooping around in our bedroom. Upshot of it was I didn't discover the loss of the ring for almost two years as I don't wear it and when was looking for it saw that it was gone. The spouse doesn't wear it, my kids are gone and the only person who had ever been in our bedroom before being run out was these so called "friends" daughter.
I haven't confronted them with it, but let it be known that in no uncertain terms that their daughter isn't allowed in our house anymore, so now they don't come over any more either and thats fine with us. In another instance when my children were your 3&4 yrs old., had a frineds daughter who use to baby sit, Well we saved quarters and half dollars in a jar over the manthle and one day I was getting them out, had to use a butter knife rather than break the bottle and my son age 4 and daughter age 3 were watching me and said dad that's how "so and so" gets them out too. When asked what it was they were talking about my son told me that she takes money from the jar and when they had seen her she threatened them said she was going to kill us and them. Well it took about two nano seconds for me to go over to her house and confront her in front of her parents. She vehemently tried to deny it, and of course her mother was taking her side and said my little children didn't know what they were talking about. This was getting rather heated until the sitters brother came in and heard what we were talking about. he must have been mad at his sister at the time (she use to treat him like crap) so he told his mom and dad that she does that in fact she had told him that she had gotten money for a movie they both went to from my money jar. Upshot of all of it was we lost her parents as friends.
I have a coniving sister in law who steals pictures from us., a while back we were looking through some of her pictures she had and talking about the old days etc., when my wife and I spied a couple of pictures we had been missing and had been heart broken over losing them. Well there they were in my sister in law's photo album, when asken how she got them she said well you guys gave them to me, I promptly told her no we didn't I remember you asking for them and us telling you we didn't have the negatives for them and t hat was the only two in existence and that no we weren't going to give them to you., they come up missing and low and behold you have them. I told her there must have been a misunderstanding on her part (family you know) and took them out of the album and took them home with us. Now it seems she is still up to her old ways. My daughter has been staying with her and her husband for awhile while working out of town and she had some jewelry come up missing and guess who she saw wearing it several months later. Once a thief always a thief i think. This sister n law has so much jewelry that she doesn't have to steal my daughters. Even her own husband remembered that it belonged to my daughter when she wore it and when she had gone to work the next day he took it and returned it to my daughter. Needless to say my daughter doesn't stay with them anymore., she can't trust her aunt.
Sorry for your loss and it especially sucks when its family that does it.