Don't know what to do

Basically, me and my partner have been together for 11 months and recently moved in together. I have 2 children 16 and 10 and he has 2 children 14 and 8, his youngest stays with us every other weekend. She stays in our bedroom and we sleep downstairs on a blow up bed. The last 4 times she's stayed jewelery and makeup have gone missing, it's not cheap makeup either and jewelry. My partner has not mentioned these things too her at all and she's the only one that's been in that room when she's stayed. This weekend she stayed again, again things went missing which I noticed before she went home so I checked her bag, in her bag was 2 of my gold bracelets, a Victoria secret body spray of mine, a Jamea Charles pallete, lipsticks and a foundation. I told my partner about this and again all he said is why is this stuff in your bag, didn't tell her off or nothing. Im getting to the point now where I don't want her staying anymore. Am I wrong to say this. She has her dad round her finger, all she has to do is pretend to cry and all is forgotten about, there's no tears she just makes a sniffling noise. I'm at my wits end. Some of my sons stuff has gone missing in the last too but never even thought to question her about it until now, really don't know what to do and could do with some advise please.