Ditto Mimi's advice.....
I grew up in a family that readily and openly accused others of stealing something if the person could not easily find the missing article. I just assumed all families operated that way.
To this day, if I have misplaced something my FIRST thought is that someone else took it. I have wrongly accused, in my mind, that my sister or husband or the cleaning lady has taken 'it.'
Thank God I have the common sense to know that my husband wouldn't steal from me. It is such a reflexive learned way to deal with loss.
So I suggest that you don't take it personally, even though they are directing their comments at you and insinuating it possibly is you. It tells you a lot about how little they trust others.
Buy them a safe, but I bet they'll not use it and still accuse others.
BTW, I am 50 next week and I cannot even count how many times I have placed a valuable item in an extra good place for safe keeping and cannot recall when the need comes where that special place is anymore. So, a lock box or safe it great answer.
I am sorry you have to deal with unsubstantiated accusations. It hurts no matter how mature and tolerant a response you choose.