Polly as others have said, your MiL cannot be discharged until there has been a full assessment of her needs and made arrangements for her care.
However, this will not stop hospitals trying to get you to agree to have her discharged to your care without the assessment and the provision of a needs package. It is perhaps useful that you and your DH dislike her so much as emotional blackmail is part of a hospitals armoury for getting patients out without assessment. I had this with an uncle.
The discharge nurse rang me up. She knew how much I cared for my uncle, how much he wanted to get out, couldn't I manage to perhaps have him home to convalesce with me for a few weeks, etc etc. Fortunately my uncle's GP practise had an excellent psychiatric nurse who had already been in touch and warned me that this would happen, so I just said, no, no, no, my uncle's discharge must follow proper procedures. In the end it was agreed he needed to move to a care home and when a place was found he was discharged.