I'd go for it if its what you want. your son can learn french quickly, and it won't do him any harm at all to have a second language if you decide to move back!
I can see the inheritance thing might be an issue - could a will sort it out? THough saying that, i've got a feeling htat property in France HAS to be split betweeen siblings .. maybe you could insure your parents in laws life for the next 5 years or something, so if they died in that period you'd get enough to buy out the business? Or could they put a clause in the will giving you the right to live in the house for a certain period after their death? Or maybe you could sign a rental agreement for the bit of the house you'd be living in, then if they died you'd be sitting tenants and couldn't be evicted? You'd probably have to contact a solicitory who knows french property law to get something watertight drawn up, but i should think it would be worth doing for the peace of mind. And make sure dh's sister knows the arrangements, to avoid bad feeling in the contingency of their deaths.
But with all watertight and legal, I'd just go for it. Why not say its a five year experiment - you can always come back.