Legally the letter should be worded so as not to be accusatory toward anyone. State that x property was located within the home and it no longer located there. During y timeframe you are aware that they were present and may have obtained the property. If any of the property was removed from the home it belongs to z and is not part of the estate. If any of these items are in your possession they are requested to be returned to y.
((Now, in my opinion, regardless of your current feelings toward your siblings $110 seems like a small amount to make a dramatic stance against. No one is perfect and obviously your siblings are not demonstrating some qualities that they may talk about, but that is their problem. I advice letting it go and hope that in the future a better relationship comes along.))

