Welcome - you're not alone.
Just to say welcome. I'm new as well (only a month or so) and my 88 year old mother, with undiagnosed but acknowledged dementia has lived with us for nearly 4 years. She has deteriorated recently and like you and your MIL we're struggling a bit: behavioural changes, almost no memory and limited understanding. I'm learning a lot - especially from fellow forum members and just reading threads that are relevant is a great support. Following a recent hospital stay (my mother became severely dehydrated) we've been plugged into local support and that is also great. The main thing is learning that many people understand what we're facing and although it's new for us, someone has always coped with it and can both understand and say it's OK to be struggling, as it's tough. So post away and I'm sure people will help (probably not much from me as I'm learning to). All the best.