Stealing from sister

Hi, i am living in USA with my husband and kids, rest of the family live oversea. Since we moved here my sister and her husband started taking things from our house in which we previously lieved. The house belongs to my inlows and they are on the first floor, the second floor we use when we go to vacation in the summer. Every summer i go i found things that are missing from the house and I see in my sister house. I asked her for couple things and she said that there were in our mom house so she took it. I knew this was not true. I did not say anything but i was sad and ashamed she lies to me so i only told my husband. My sister was doing some things for my kids like take them from the airport, drive them to the airport and e.c. She seemed so sincere and loved my kinds so much so my hearth did not allow me to say anything. But last summer my in lows told me my sister husband wend with a truck and loaded building matherials from our back yard 2 or 3 times. I get very mad and wrote a e mail to my sister. I told her i do not like her or her huband going to our house with not me or my husband. She did not response to the e mail or contact me with any explanation. Resently my mom told me i am not right to blame her about stealing things from me. I do not know what to do. My sister husband even stole things from my mom property couple years ago. My mom said it was not my sister it was her husband. She said i know your sister and she will never steal from you. I do not know what to do. I am constantly thinking about this and i can not sleep at night. My sister was the favorite child of the family when we were kids. I do not want to talk to my mom. I feel so lost and have no idea what to do. If anyone can help me with a sugestion.