To be uncomfortable with sil looking after baby alone

[UncomfortableSIL]

Hear me out first, sil has bought a baby gym etc and insinuated she will be looking after our new baby a lot at her house to help out. Sil has always wanted another child but can't, she has a disabled ds who can be aggressive with autism and doesn't discipline him at all.
This ds has pica and also has eaten things like magnets, marbles you get the gist..

Sil has been saying that her ds loves babies and is obsessed with the baby wants to see them etc. She has suggested having baby alone at her cramped apartment to give us a break which also had lots of hazards, is not baby proof.
I am not comfortable with sil having my baby alone there with her ds.. I don't trust her to keep an eye on him or discipline him if he is being rough with the baby. All it takes is for her to leave the room for one minute.
How do I word this diplomatically without upsetting sil? I have a good relationship with her and don't mind her or her ds being around baby but only feel comfortable if I am there to supervise.

It is fine to say no she sounds over excited, over enthusiastic and over stepping but you don't have to give your baby to anyone.

You just have to keep distracting her by saying maybe when baby is older.

When she offers to have baby, just say we don't need a break thank you but you're welcome to come and visit us. And keep repeating. You don't have to say anything about her son, if you do I can guarantee that it will cause a rift. Just reiterate that you're happy looking after your own baby and don't need or want a break.

we don't need childcare
No thanks!
No, it's ok, we won't need childcare for a few years yet

It's fine if she gets upset, that's a her-problem to figure out, don't give it a moment of thought.

You can thank her for the offer but say that you aren't ready for baby to go with anyone solo but look forward to meeting up together (if that feels OK?).

No need to say anything negative, she offered something that you don't want.