Sons grandma wants to take him over night...

So my son is 2 months old and my mother takes him once a week over night. I’m okay with this because I trust her, she raised me and my sister and we came out good and she never hurt us or put us in danger. Well my boyfriends mom texted me today asking if she can take my son Friday night to go see his uncles on Saturday. She has not once taken him even for a few hours and just reading that text made me so sick to my stomach. My boyfriends mom lost custody of all four of her kids forcing her two oldest children to never be adopted and live in the foster system. Her oldest son who is my boyfriend was shown drug dealing, and killing people at a young age. He’s never killed anyone but he did call drugs as young as 9 years old. Not to mention she let him smoke weed at 6 years old and he’s been smoking ever since. She also let his biological father beat him, burn him with cigarettes, and molest him. When he told his mom she didn’t believe him. But when he was 9 before his dad did anything to him his moms girlfriend raped him! AT 9 fucking years old. Then when my boyfriend was 18 he was living in his first apartment and reconnected with his family. Well there was a time when they were all together and he finally told his mother and god sister who happens to be his rapists sister that she raped him and not even a month later she’s was around! They let her around the children in the family and him after he said he was raped by her! FUCKING SICK.

Well me being me I get anxiety just by telling people no or making people mad at me especially his family. So I told her I’d talk to her son about it. I told him no I don’t want her taking my son and he said he was waiting for her to ask since my mom takes my son. I told him to be the bad guy and tell her no if she wants my son to see his uncle we will bring him he is not staying with her. I just don’t know how to tell her no. I’m in a way afraid she’ll get mad then talk shit and post it on social media that I won’t let her take my son and then tell his god sister who will then come at me all crazy. My mother never willingly put me and my sister in harms way! My sister and I were actually never harmed because my mom was so cautious in who she let in our lives. If we didn’t like a boyfriend my mom had she’d break up with them right away. His mom was too worried about sex and stuff that she put her son in harms way and let her son be hurt. I’m not that mother. My son will NEVER go with her.

How do I tell her this nicely though so she doesn’t hate me. Or start drama.