As for the loft and the office, I'm a little confused - are they part of the house? Separate from it? Either way, does sil now own them? If she does, it would seem to me it's her job to get them cleared out, even if she has to hire help.
Or is it something you were supposed to help take care of as an executor? I agree with Howcome that you need to check out where you stand legally. I don't want to worry you either, but this is important.
If you find you're at all responsible, it might be worth hiring people, if you can afford it, to help get things out of the loft so you can sort through them. Really, imo, all 3 executors should share the cost of the help and go through the items. But idk if that would work given the tensions involved.
If you're not legally responsible for anything, then I agree with following MawBroon's and FN's advice. I would add, you shouldn't have to be around this woman you dislike so intensely, if you don't have to.
Hoping this episode is behind you soon.