My MIL and DS

Hi everyone - apologies that this will be long. And also for posting here as I'm not a Gran, I'm a young mum but trying to look at this situation from the other side, as it were, so hoping to find some advice on here. I've posted about this problem on mumsnet ages ago, so if you also frequent the relationships board there it may ring some bells. It's not really been fixed though and frankly I have no idea how it can be:

Basically, DH believes, but his memories are too indistinct to be certain, that his mother sexually abused him as a child. He has never been certain enough to either confront her or cut her out of his life, as he basically doesn't trust himself, and says if he's wrong/being crazy it's too unfair. He now has what I would call a decent if distant relationship with her. We see her every few weeks, and they get on quite well on a superficial level.

Personally, I am inclined to think it did happen, for lots of reasons but perhaps most pertinently that DH's older brother is no contact with MIL, and has told DH he will explain once she's dead. BIL also caught me on my own the last time we saw him (we see him about once a year) and asked me out of nowhere if MIL ever babysitS DS, who's 15 months now and was about 5 months then. I said no, bil just said 'thats a very good thing', stared at me intently, and walked off.

And that really is the crux of the issue. She is DESPERATE to babysit DS, and is constantly asking. We just keep deflecting, and as far as she's aware we've never left him with anyone except nursery (actually my Mum's looked after him for a couple of hours a couple of times, and FIL - her ex- comes round quite regularly to take him to the park etc, but she'd go ape if she knew). Obviously we can't let her, the risk is too great - but I have no idea how to keep deflecting. And I feel like such a bitch if we/DH are wrong and it's his mind playing tricks.

Aside from the major issue, I don't particularly like her, and am not sure I'd trust her with DS overnight (as she regularly drinks and drives, for instance, and sees no issue) but wouldn't want to block their relationship to this extent.

I just don't know how to go forward. I try to facilitate her seeing him as much as possible (far too bloody much while I was on ml) in case DH is wrong, while NEVER leaving them alone I'm case he's right.

Is there anything I can do to handle this better? I don't want to be an awful DIL to a quite possibly innocent woman but I have to protect my son!