Hello and welcome
@SallyC85 to this friendly and supportive forum. There is a wealth of shared experience of dementia here so I am glad you have found us.
I am sorry to read about your mother in law's recent diagnosis. It's tough to get such a diagnosis, even, as you say, you did know there was something.
The following link may help for some information:
Alzheimer’s disease is a physical illness which damages a person’s brain. This eventually causes dementia. Alzheimer’s disease is the most common cause of dementia.
www.alzheimers.org.uk
Please do have a good look around the forums and ask any particular questions you may like to. Members here really do want to help.
The only practical matter I would advise you do sooner rather than later, (if not done already) is for your husband and sister to organise Lasting power of attorney. This will make things a lot easier further down the track, because as you will know already, Alzheimer's is a progressive disease, so now is the best your mother in law will be.
How to make a lasting power of attorney (LPA): starting an application online, choosing an attorney, certifying a copy, changing an LPA.
www.gov.uk
It can be so overwhelming for family to take in... so take your time please and do let us, the forum members know, if there is anything we can particularly help with. Take care.