no you can't take my two month old

My MIL ask me at least twice a week if she can come pick up my two month old son for a week so we can "have a break"! She lives 4 hours away and every time I nicely decline but I am getting so tired of her asking! When her first son was born she gave him to her mom to raise but now gets mad that he is 23 years old and doesn't treat her like the mom. I am sorry but I am not going to just hand over my son when I "need a break". I don't know how to tell her and/or my husband that it's not going to happen and it's getting annoying! However, when he is 7 months my husband and I are going on a cruise and my mom will be driving up here to stay with him at our house. So I feel like my MIL thinks since that's planned that it's okay for him to jus take him for a week!