My SIL is stealing us and I/We don't know how to deal with this

Hi all!
I 29F My partner 35M My SIL 29F
My husband and I live with his daughter (6F) he has full custody and I raise her like a daughter. We are really happy together and everything is great. My husband and his family are from another country and his mother was living with us till March when she came back to their country. The kid goes to school since 13 PM to 17PM so we were going to hire a nanny because we both work full time but my SIL was unemployed and had nowhere to live so she asked if she could live with us and take her to school till she finds a job, she only has to take gee to school, she was selling things online as an income. One month after she moved we noticed some things were missing, like a video graphics card for PC very expensive, some tools my husband used for job and some money. We had an employee who used to come once a week to clean all the house, we fired her thinking it was her fault (we paid her everything and we never told her why we took that decision). Times passed by and I started to notice this: My BC pills missing My creams, clothes missing too
I swear I thought I was insane, like I was losing my memory but one day my husband was off and when I ask why he tells me he thinks his sister is stealing us, same situation, money disappeared from his wallet and he was thinking he was losing his memory but now it's a lot of money and there are another tools missing (expensives too). Then I remembered once the kid tells me "my aunt took money from your wallet" and SIL acts surprised and says "it was because I wanted to buy eggs". So everything leads to the same path.
We don't know what to do, we are very angry because we both work A LOT, we are stressed everytime now, hiding our things in OUR house because of her. I am MAD. But he talked to her and she denies everything (of course). We discovered she is dating a recluse who sees once a week in prison. My husband is afraid that if we ask her to leave she is going to end in the streets but she is not looking for job, not selling things online anymore, she only lies in the bed watching TV. We are both very stressed and he wants to tell his mum about this but he is afraid this is going to wreck the family.
Any advice?
TL;DR: SIL stealing money and things but we don't know how to approach it without wrecking the family.


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