lock up all your valuables. For things like wallet, watch, mobile phone etc after you return home, put it in a safe place inside your bedroom and ask her not to go inside your bedroom when you return home.
Find a replacement maid and do not renew her contract after her current contract expires. However, in the meantime, better have somebody at home all the time when she is in and if not possible install a webcam and tell her that you want to see the baby when you are out as your sister-in-law no longer with her to help taking care of baby. Your maid will know what you mean and hope she will behave.
You have one and only one solution. That is terminate your domestic helper in your convience. I think no one will torlent a bad conduct rather than a bad performance domestic helper. Good luck!
I do not suggest you 報警備案. You know, once your break the relationship with the domestic helper, I am so worry about her performance and how does she treat on your baby when you and your spouse not at home. You know what I mean?! Of course, she dare not to hurt your baby or whatelse. BUT your family's FOOD are prepared by her. She is the one taking care of your baby. [Just my personal comment]