okay I need help guys (grandparent thinks my daughter is hers)

So my fiancées mom watches our daughter when I am at work during the day since I go in a little later than my fiancé does and he gets off earlier and picks her up, but here lately she seems to think my daughter is hers.. every time I put clothes in her diaper bag/ burp wraps and bottles and bibs and toys and hair accessories and shoes his mom will keep all of it and not return it to us. I have ask politely several times for her to put her stuff back in her diaper bag and if she needs anything I'd gladly buy her stuff to keep over at her house( I have already bought her a few bottles and some clothes and I supply her with the formula I buy her 3 cans a week to keep at her house and when she gets low on it her more on top of the formula for our house.) she just turned 2 months old today and his mom yesterday took her to a photographer to get 2 month pictures done for herself and so she can put them in her picture album for her house and she said if I want prints of the pictures I'd have to talk to the photographer myself. And she did this at 1 month to. I work a lot as I am a server/bartender/key manager/trainer coordinator and I finally had a full day off and I was spending it with my daughter and I had to take my car to the shop to have them look at it and she called me while I was and said my daughter didn't need to be there and said she would come pick her up, I told her no it was fine and I wouldn't be there much longer and she showed up anyways to pick her up. Any time she thinks we can't handle watching her she just comes and gets her. And last night when me and my fiancé went to go pick up our daughter from his parents house we asked for her stuff back and I told her she could keep some of it but we need a lot of it back because we only have a hand full of outfits left because she had them all and she asked us why we needed it back and gave us an eat shit a die look when we started putting her stuff in the diaper bag. And she looked and me and said you looked tired why don't you just leave her here for the night so I can take care of her properly since I'm not tired. We have talked to her several times and nothing seems to work. I just need advice on new ways to try and talk to her.