Boyfriends cousin

Jessica

My boyfriends cousin just recently had a baby and she let's her aunt and uncle watch her all the time. She is less than 6 months of age. I am not saying that isnt okay I just personally dont want my child spending the night multiple times a week or month with anyone else. My MIL has already made some comments that point to her possibly thinking it's going to be the same way with me. This is her first grandbaby and I dont want to sound rude or even really have to deal with telling her no. I told my boyfriend since it was his mom he was gonna have to let her know. He can be alittle to the point when he talks to his parents without having a conversation and I just feel like it's not going to be discussed properly. How do I approach this situation gracefully ?