Firstly, as per the previous comment, I think the most urgent thing is to get him to his GP for some tests, diagnosis and medication. Two years ago my father was working full time but has just moved into a permanent care home, and I do think that we missed the early signs and didn't act quickly enough.
Experience has also taught me that the worst person to care for someone with dementia is someone who doesn't feel able or willing, so for the benefit of everyone concerned it definitely doesn't sound that it should fall to you.
A friend recommended a company to me called Home Instead - they provide a broad range of care, support and companionship that can be scaled up or down depending on what you need. Unfortunately it wasn't something that we were able to pursue for Dad but, from the sound of your circumstances, it's definitely worth exploring.
http://www.homeinstead.co.uk
I can't emphasise enough how important it is to encourage the family to get your father in law to his GP for diagnosis and medication asap though.