Good luck, hope it goes well. I'm absolutely not going to flame you for not wanting her to live with you (I came to same conclusion about my Mum after posting on here and hearing opinions, everyone said don't do I).
Just wanted to warn you to be very clear in your mind what role you will be prepared to take when the Social Worker comes eg you can help with her finances, moral support but nothing practical. Has she been referred to the Memory Clinic yet ? Also you'll need to apply for Attendance Allowance if she doesn't have it. Get some help filling this in (Alzheimer's Association referred me to a charity) as they are turning loads down at the moment.Age Concern can help too I think . You'll also need to get a Power of Attorney done if not already, as long as she has capacity to do so. You can do this yourself,I did most of it but same charity helping with AA is helping with this for much less than a solicitor.
Social Services will be more concerned with personal care rather than the house. Make sure they know about the continence issues .In our area you need to be deemed to have Substantial or critical need for a care package. My Mum has one due to her legs rather than because of her memory but I think once she's gone to see consultant at Memory Clinic for Brain scan results they will take over. Initial SS team was the Reablement team, now she's under The Long Term team and then ultimately will be the Mental Health Team.
Social Services will probably want to do a financial assessment at some point so try to get to the bottom of her finances if possible and find out where up to date statements are. Mum has a cleaner (cleaner number 4 so took a bit of tryng o get right one !) who makes sure place as up together as possible. There are a fair few people who deliver food, either ho or to be heated in a microwave. In our area there's someone called the Memory Advisory Service who can help with Memory Aides and safety.
Medication can be deliverd by the local pharmacy in blister packs once set up by the GP. I think there are usually Continence Advisory Services that can be accessed via the GP.
I know this must be incredibly hard to deal with especially with the worry of your BIL. It will be especially hard for your DH. This is where you come in I'm afraid, to help stand firm and make sure your MIL gets the care she needs without that meaning you and your DH physically doing it. It's very difficult to actually say 'I'm not going to care for her' whn faced with it. Follow up SS visits with emails. Sorry that's lots of info, apologies for bombarding you.