Is it wrong of my husband and I to not want my sister to watch our son?

I am having a dilemma and it has been causing me a lot of anxiety worrying about how to handle this situation. My boyfriend and I don’t feel comfortable with letting my sister ever watch our son, and my boyfriend is extremely adamant about it.

My sister and I are close, but she has said and done some things in the past that makes my boyfriend, and honestly myself weary of her.

We have witnessed her yell at her 1 year old son to “shut the f**k up” when he was crying. She made a comment about my son that if she had to deal with him crying the way he does (he’s colic) that she would’ve smothered him if it were her son. She’s also made a comment in the past about how when her son was crying she thought about putting a pillow over his head…
She also makes weird comments about my sons genitals, how it’s long. I put diaper rash cream on him and she thought it was funny to say “oh yeah mom touch my balls” and it made me extremely uncomfortable that she would say that and basically sexualize something as simple as putting ointment on my baby’s butt.
I know she wouldn’t hurt my baby, but we also aren’t comfortable with her watching him by herself because of the way she’s treated her own son, and the things she’s said and done. We feel like if she can treat her own son badly, then what would stop her from treating ours the same or worse, especially if he’s crying and she can’t handle it? how am I supposed to tell her that she can’t watch him? Are we over reacting? She acts loving and caring towards my son whenever she sees him, she just says weird things. I am having severe anxiety about it, I’m really lost on how to approach this if it ever comes up, but thankfully it hasn’t been an issue yet because he’s only a month old. But I’m dreading the day that I have to approach it and I really need some advice on how to handle this situation or if I am overreacting. Thanks